UNIVERSIDADE ESTADUAL DE CAMPINAS
Instituto de Física “Gleb Wataghin”
LEANDRO DAS MERCÊS SILVA
ELECTRONIC TRANSPORT
IN SEMICONDUCTING MOLECULAR ENSEMBLES
TRANSPORTE ELETRÔNICO
EM CONJUNTOS MOLECULARES SEMICONDUTORES
CAMPINAS 2018
EM CONJUNTOS MOLECULARES SEMICONDUTORES
Tese apresentada ao Instituto de Física “Gleb Wataghin” da Universidade Estadual de Campinas como parte dos requisites exigidos para a obtenção do título de Doutor em Ciências.
Thesis presented to the Institute of Physics “Gleb Wataghin” of the University of Campinas in partial fulfillment of the requirements for the degree of Doctor in Science.
Supervisor/Orientador: Prof. Dr. Carlos César Bof Bufon Co-supervisor/Coorientador: Prof. Dr. Antonio Riul Júnior
ESTE EXEMPLAR CORRESPONDE À VERSÃO FINAL DE TESE, DEFENDIDA PELO ALUNO LEANDRO DAS MERCÊS SILVA, E ORIENTADA PELO PROF. DR. CARLOS CÉSAR BOF BUFON.
CAMPINAS 2018
CAMPINAS 2018
MEMBROS DA COMISSÃO JULGADORA DA TESE DE DOUTORADO DE LEANDRO DAS MERCÊS SILVA – RA 81900. A TESE FOI APROVADA APÓS SUA
APRESENTAÇÃO AO INSTITUTO DE FÍSICA “GLEB WATAGHIN”, DA
UNIVERSIDADE ESTADUAL DE CAMPINAS, EM 19 DE MARÇO DE 2018.
COMISSÃO JULGADORA:
- Prof. Dr. Carlos César Bof Bufon – Orientador – CNPEM - Prof. Dr. Lucas Fugikawa Santos – UNESP/IGCE
- Prof. Dr. Adriano Reinaldo Viçoto Benvenho – UFABC - Prof. Dr. Luiz Fernando Zagonel – IFGW/UNICAMP
- Prof. Dr. Eduardo Granado Monteiro da Silva – IFGW/UNICAMP
OBS.: Informo que as assinaturas dos respectivos professores membros da banca constam na ata de defesa já juntada ao processo de vida acadêmica do aluno.
CAMPINAS 2018
To my grandpa, Ademar.
“Worker bees can leave. Even drones can fly away. The Queen is their slave.”
Acknowledgments
This work was carried out during the years 2014-2018 at the Brazilian Nanotechnology National Laboratory (LNNano), in the Brazilian Center for Research in Energy and Materials (CNPEM), and at the “Gleb Wataghin” Institute of Physics (IFGW), in the University of Campinas (Unicamp), both in Campinas/SP, Brazil.
I owe my most profound gratitude to my advisor, Professor Carlos César Bof Bufon. Without his continuous efforts concerning this work, the present study would hardly have been completed. His guidance into the electronic transport in molecular systems and his supervision in sample fabrication and electrical measurements also contributed to make this work complete. I express my warmest gratitude to both my co-advisor, Professor Antonio Riul Junior, and my co-worker Dr. Rafael Furlan de Oliveira, who guided me through the good discussions regarding everything, from device electronic transport and dielectric properties of molecules to red tape things and concerns about life.
I am deeply grateful to Professor Henrique Leonel Gomes, from the University of Algarve (UAlg, Portugal), for the co-advising in understanding the electronic features presented by microscale electronic devices, and Mr. Davi Henrique Starnini de Camargo for the learnings regarding the fabrication of nanoscale transport junctions.
I also want to express my gratitude to my colleagues from LNNano, Maria Helena Piazzetta, Ângelo Gobbi, Evandro Lanzoni, Carlos Costa, Érico Teixeira-Neto, Leirson Daniel, Ricardo Magno, Felipe Marques, Luis Falsetti, Carolina Lopes, Paula Petrini, and Rudolf Francesco, who were involved in some manner with the works I participated during this PhD.
I acknowledge CNPEM, Unicamp, UAlg, and the founding agencies that made this work possible, namely FAPESP (2014/25979-2), CAPES, CNPq, and SibratecNANO.
I also want to thank my dear friends, Lucas Dantas, Érick Colato, Ivania Silva, Guilherme Balieiro, and Renato Lima, for the assistance and friendship. And finally, for the absolute support and love, I owe my last but meaningful thanks to my parents, Roberto and Ioniras, my brother Bruno, and my better half Letícia Mariê.
de interações quânticas entre portadores de carga e moléculas, que resultam em diversos fenômenos físicos e químicos. Seu entendimento tem aberto caminho para o desenvolvimento de conceitos inovadores de dispositivos, como transistores orgânicos, diodos orgânicos emissores de luz e biossensores. Para tais aplicações, a classe de conjuntos moleculares semicondutores (do inglês, semiconducting molecular ensembles – SMEs) desempenha papel fundamental. Nesse cenário, o presente trabalho investiga as propriedades de transporte eletrônico em SMEs. Para esta tarefa, junções de transporte foram empregadas para conectar estruturas moleculares de forma confiável na micro/nanoescala. Na microescala, o transporte eletrônico demonstrou sua alta dependência de cargas espaciais extrínsecas aos SMEs, capazes de regular a condução em dispositivos conectados por eletrodos inferiores. Assim, o papel da arquitetura do dispositivo na condução de SMEs com até 500 nm de espessura foi desvendado na perspectiva inédita do transporte eletrônico. Na nanoescala, nanomembranas foram utilizadas para conectar conjuntos finos e ultrafinos de moléculas em junções moleculares, com espessuras que variam de 5 a 60 nm, integrando-as aos processos convencionais de microfabricação. As investigações revelaram uma profundidade de ~ 20 nm para o transporte eletrônico coerente nos SMEs, e o tunelamento sequencial (do inglês, sequential tunneling –
ST) foi demonstrado para moléculas adsorvidas fisicamente pela primeira vez na história. Tanto
a fabricação/caracterização de dispositivos eletrônicos baseados em conjuntos moleculares, quanto suas aplicações envolvendo frutos destas investigações – isto é, um transistor chaveado na água, um dispositivo de carga individual e um sensor de pressão – estão demonstrados neste trabalho. Neste sentido, as descobertas aqui apresentadas contribuem formalmente para unir a eletrônica orgânica à eletrônica molecular considerando o conjunto elementar de moléculas semicondutoras adsorvidas fisicamente.
Abstract
The electronic transport in molecular ensembles is intrinsically related to a series of carrier-molecule quantum interactions, resulting in embracing physical and chemical phenomena. Its understanding has paved the way for the development of novel device concepts, such as organic transistors, organic light-emitting diodes, and biosensors. For all of these applications, the class of semiconducting molecular ensembles (SMEs) plays a needful role. In this scenario, the present work investigates the electronic transport properties of the SMEs. To this task, micro- and nanoscale transport junctions were employed to reliably connect the molecular structures. At the microscale, the electronic transport demonstrated its high dependence on extrinsic space-charges that rules the conduction in bottom-electrode devices connecting 50-500 nm thick molecular ensembles. The role of device architecture on the SME-conduction was unveiled from the unprecedented perspective of charge transport. At the nanoscale, nanomembranes were used to reliably connect thin and ultrathin molecular ensembles (from 60 to 5 nm), integrating them to the standard microfabrication processes. The charge transport mechanisms have revealed a coherence depth of ~20 nm in the SMEs, and the sequential tunneling (ST) was firstly demonstrated for the physisorbed molecules. The fabrication and the characterization of electronic devices based on molecular ensembles, as well as applications involving the proposal – namely, a water-gated transistor, a single-charge device, and a pressure sensor – were also demonstrated. Hence, the finds formally presented here contribute to bridge the gap between molecular and organic electronics for the elementary set of physisorbed, semiconducting molecules.
a – radius of the sphere A* – effective Richardson
constant
A1 – geometrical parameter
AFM – atomic force microscopy
Ageo – geometrical contact area
of the VATJ
Ainj – effective injection area
of the VATJ
Ainj(i) – effective injection area
with no-load applied in the VATJ
Ainj(ii) – effective injection area
with a load applied in the VATJ
Ainj(0) – effective injection
area as a function of 0
B-Au – bare Au BC – bottom contact
BSE – backscattered electrons
C0 – self-capacitance
CB – Coulomb blockade
Ci – electric double layer
(insulator) capacitance CNPEM – Brazilian Center for
Research in Energy and Materials
CuPc – copper phthalocyanine Cys – cysteine d – Debye length D – drain dI/dV – differential conductance DNTT – dinaphthothieno-thiophene DT – direct tunneling E – electric field e – elementary charge E(i) – transition electric field
situation (ii) (Fig. 3.1.4b)
Ea – activation energy
EA – electron affinity EC – charging energy
Eext – external electric field
EGaIn – euthetic GaIn Eq. – equation
Er – electric field component
along r
Er,y – electric field component
along both r, y
Er,y(y) – electric field
component along both r, y as a function of y
ETFL – E(ii) for the microscale
devices
Etr – E(i) for the microscale
devices eV – electron Volt Fig. – Figure FN – Fowler-Nordheim G – gate GST – enzyme glutathione S-transferase GST – tripeptide reduced glutathione h – Planck’s constant. HF – hydrogen fluoride HOMO – highest occupied
molecular orbital
I – electric current I(E) – electric current as a
function of E
I(i) – electric current with
no-load applied in the VATJ
I(ii) – electric current with a
load applied in the VATJ
I(V) – electric current as a
function of V
I(0) – electric current as a
Child’s regime
IDS – electric current between
S and D electrodes IFGW – “Gleb Wataghin”
Institute of Physics
Ihopp – electric current for the
hopping condution
IOhm’s – electric current at the
ohmic regime
ITFL – electric current at the
TFL regime
J – current density J(E) – current density as a
function of E J(T-1) – current density as a function of the 1/T J(z) – current density as a function of z. JPCC – The Journal of Physical Chemistry
Jy – current density in the
y-direction
k – Boltzmann’s constant Kgeo – constant related to R1/2
in the VATJ
Kinj – constant related to Kgeo
in the VATJ
L – separation between the
metal electrodes of the microscale devices LNNano – Brazilian
Nanotechnology National Laboratory
LUMO – lowest occupied molecular orbital
m* – effective electron mass m0 – free electron mass
MolEl – molecular electronics n – any natural number
ensemble LUMO
n0 – concentration of free
charge carriers in thermal equilibrium
nBC
Child’s – n for the Child’s
conduction in BC devices nBC
Ohm’s – n for the ohmic
conduction in BC devices nBC
TFL – n for the TFL
conduction in BC devices
NI – impurity density
NLUMO – density of states in the
LUMO level NM – nanomembrane
Nt – trap density
nTC
Ohm’s – n for the ohmic
conduction in TC devices OFET – organic field-effect
transistor
OrEl – organic electronics
P – applied load
pH – potential of hydrogen
q – electronic charge of
carriers
Q – total charge of the SCR q(r) – SCR charge distribution r – radial coordinate
R – tube electrode radius in the
VATJ
RMS – root mean square
RQ – quantum resistance RT – resonant tunneling RT – tunneling resistance S – average sensitivity S – source S1 – pre-exponential constant
dependent on the tunnel barrier height
S2 – exponential constant
dependent on the tunnel barrier height
SAM – self-assembly monolayer
SCD – single charge device SCLC – space-charge-limited
current
SCR – space-charge region
SE – secondary electrons SED – strain energy density SEM – scanning electron
microscopy SiOH – silanol
SMAX – maximum sensitivity
SMEs – semiconducting molecular ensembles ST – sequential tunneling STM – scanning tunneling microscopy T – absolute temperature t – ensemble thickness TC – top contact
tCuPc – CuPc ensemble
thickness
TFL – traps-filled limit tMolEl – transport distances in
MolEl
tOrEl – transport distances in
OrEl
Ttr – transition temperatures
Unicamp – University of Campinas
V – voltage bias
V(y) – electrostatic potential as
a function of y
v/v – volume/volume ratio VATJ – variable area transport
junction
VDS – voltage applied between
S and D electrodes
VGS – voltage applied between
G and S electrodes
Vs – surface electrostatic
potential
VTH – threshold voltage
W – electrode width of the
microscale devices
x – abscissa coordinate y – ordinate coordinate y0 – diameter of the insulating
sphere
z – coordinate perpendicular to x, y
I – electric current variation
P – applied load variation
tCuPc – deviation of tCuPc
– mean hopping distance – attenuation factor (V) – attenuation function DNTT – attenuation factor of DNTT ensembles – diameter compression 0 – intrinsic diameter compression – ensemble dielectric constant 0 – permittivity in vacuum r – dielectric constant – field-dependent activation energy
B – potential barrier at the
metal/SME interface T – trapping potential
– carrier mobility – frequency of electron
thermal vibration at the trapping centers
– ratio between free and total carrier densities – electrical conductivity BC – electrical conductivity of BC devices TC – electrical conductivity of TC devices
Resumo Abstract
List of abbreviations and symbols
1 Introduction ... 13
2 Motivation (state of the art)... 17
3 Theoretical background ... 18
3.1 Electronic transport mechanisms ... 19
3.2 Connecting molecular ensembles ... 29
3.3 The nanomembrane-based transport junction ... 33
4 Electronic transport in microscale devices ... 36
4.1 Experimental details ... 37
4.2 The role of device architecture on the electronic transport ... 39
4.3 Water-gated transistor based on bottom-electrode devices ... 52
4.4 Conclusions ... 59
5 Electronic transport in nanoscale devices ... 60
5.1 Experimental details ... 61
5.2 Long-range coherent tunneling ... 65
5.3 Single charge effects ... 76
5.4 Pressure sensor based on variable-area transport junctions ... 84
5.5 Conclusions ... 95
6 Summary ... 96
References……….. 97
Introduction
Technological development and measurements of electronic transport across single-molecules, monolayers, and molecular ensembles have magnified the frontiers of molecular electronics (MolEl). Extensive research and numerous experimental paradigms have been employed to elucidate how structure controls the electronic transport of aromatic, alkane, and oligomeric molecules.[1–4] Ideally, a single-molecule is a zero-dimensional (0D) object – with dimensions smaller than the electron wavelength.[5] To measure a single-molecule, the separation between the electrodes is commonly less than ∼5 nm, and the conduction effectively occurs across 0D/1D pathways. A monolayer, which has two spatial dimensions (2D),[5] still preserves the vertical electrode separation around 5 nm and the 0D/1D transport. The molecular ensembles, finally, comprise the scenarios where the transport may happen over tricky molecular pathways (> 1D) – along tens of nanometers, such observed for molecular wires[6,7] and small-molecule thin-films.[8–10] In comparison to the 0D/1D, the transport through molecular ensembles may not be understood as a rough superposition of manifold, parallel/series, single-molecule/monolayer conduction events.[5,11] Hence, the precise control of the electronic transport by variations in both the molecular stacking and distribution has brought out a unique appeal to the development of novel and functional device concepts for MolEl.[8,12] Furthermore, join to the numerous existing molecules, such core aspects make the MolEl a puzzling but consolidate route to the future nanoscale-integrated electronics, from the design to the conception.[2]
From the perspective of molecular functionality, there are two major pillars which hold the key features in MolEl: intrinsic properties of single-molecule,[2] and collective effects of molecular ensembles.[5] Intrinsic properties have been commonly demonstrated for single-molecules, echoing to the molecular ensembles where the confinement of the electron wave function is not effective in all of the three dimensions.[5,8] Consequently, lateral quantum interactions between molecules may result in additional electronic properties referred as ensemble collective effects.[5,10] Such effects are known to be orthogonal to the quantized ones and shed some light on the emergence of the macroscopic charge transport. Biochemistry, biophysics, and thin-film organic electronics are just the mainstream demonstrations of collective molecular effects. Going through cumulative effects is a current technological need, and the molecular ensembles do offer a reliable way to manipulate each one of the cumulative contributions of molecules up to the nanoscale.[5]
ensembles have been widely used as active material for organic field effect transistors,[13] diodes,[14] and sensors in general.[15] There is vast literature showing that the transport mechanisms in such structures are dependent on how strongly the molecule couples to at least the injecting electrode.[12] Researchers also agree about the dominance of hopping conduction across sub-10 nm thick semiconducting molecular ensembles (SMEs).[5,10,16] Despite such a consensus, however, measurements of transport ratios across molecular structures are known to present an persistent combination of contradictions,[10,17] mostly due to the misleading correlations between the geometrical and the effective electrical contact areas in transport junctions.[5,17] Notice that the SMEs commonly form large-area transport junctions, where the geometrical contact may often exceed values of 1 m2. For organic electronics (OrEl) devices, such areas are practically the same as the injection ones. However, for MolEl devices, with typical few-nanometer transport distances, the effective contact occurs only through the asperities distributed over surfaces involved – i.e., electrode and ensemble surfaces.[10] Furthermore, for the MolEl scenario, only a fraction of the effective, physical contact region is conductive,[10] and the electrode roughness becomes critical for both the molecular ensemble uniformity[18,19] and the measured transport properties.[20–24] For instance, estimations of the effective contact, from measurements of surface adhesion and friction, indicate discrepancies of ~1,000,000 % between the geometrical contact areas and the effective ones, depending on the hardness of the materials, their topographies, and the loads applied to the contacts. Although well-known, the effective contact issues/features remain unexploited in MolEl.
Motivated by such scientific and technological need, innovative strategies to investigate molecular systems have been developed, involving from theoretical models[25–27] and computer simulations[28,29] to pioneering experimental architectures.[17,30–32,16] As the foremost outcomes, coherent tunneling and activated hopping have emerged as the mainstream concepts capable of describing the short- (~1-10 nm) and the long-range (> 10 nm) electronic transport across molecules and molecular ensembles, respectively.[9,16,33] Additionally, other specific conduction mechanisms such as Frenkel-Poole emission and field-assisted tunneling have also been demonstrated in transport junctions for ensemble-MolEl.[16]
At the nanoscale, direct tunneling (DT) currents are expected to scale exponentially with length, according to the attenuation factor β. The β-value is calculated as the current density (J) decay observed in the J-t plot, where t is the transport distance. The literature reports β-values of ~2-3 nm-1 for aromatic molecules,[34–41] and 8-9 nm-1 for aliphatic ones.[21,42–44] Smaller values of β have also been reported, eventually less than 1 nm-1,[45,46] and have both technological[47,48] and fundamental[49–52] consequences. Such values reflect the possibility of carrier tunneling across distances longer than the molecular length – an attractive appeal for the development of complex MolEl. The β-values smaller than 1 nm- 1, for example, are often demonstrated as temperature-dependent, and have been attributed to hopping conduction across molecular pathways.[37,53] However, recent works have shown that both the molecular orbitals energetically close to the contact Fermi level,[37] and the strong electronic coupling between π-electrons of neighboring molecular sites[28] can lead to delocalization of carriers. Such a possibility has revealed an interesting contribution of coherence even inside the activated regime, which manifests exhibiting a minimal temperature-dependence.[9,28,33] Furthermore, the reported observations of coherent charge transport across long distances (at least several nanometers) have consequences for a variety of quantum effects never found outside the molecular scale – e.g., the nearly ideal Hall effect and the positive magnetoconductance.[54]
The electrical response of molecular ensembles can also be evaluated using conductance measurements.[2,4,5] By varying the electric field in the junction and/or the temperature of the reservoir, one can determine the characteristics of the electronic transport (e.g., the dominant mechanism, the involved energies, and timescales) across the molecular ensembles. The transport characteristics at the nanoscale are recognized to be strongly dependent on the coupling between electrodes and molecules, [3] as well as on the involved interfaces, especially for large-area junctions.[5] The understanding of how charge carriers flow across the junction, the contribution of the electrodes, interfaces, and orbitals, are essential to determine the properties and possible applications of the molecular ensembles. In the present doctoral thesis, prototype organic semiconductors, namely the copper phthalocyanine (CuPc) and the dinaphthothienothiophene (DNTT), are employed to fabricate transport junctions and evaluate the electronic transport mechanisms of their respective SMEs – with thicknesses varying from few to hundreds of nanometers. The Au/SME/Au transport junctions were evaluated in two configurations, as illustrated in Fig. 1.1: at the left-hand side, a planar microscale device designed to minimize interfacial contact effects is shown; at the right-hand side, a vertical,
Figure 1.1: Illustration of the device platforms employed to investigate the electronic transport of SMEs in the present work. On the left-hand side, a picture of a glass slide, held by a hand, containing five planar Au/SME/Au transport junctions, is exhibited. Such architecture is typical of microscale devices, and is discussed in detail in Chapter 4. On the right-hand side, the schematics (computational image) of a vertical Au/SME/Au transport junction based on nanomembrane is depicted. This device allows the investigation of the transport mechanisms at the nanoscale (detailed in Chapter 5).
nanomembrane-based junction designed to reliably connect the SMEs at the nanoscale is drafted. The micro- and nanoscale device architectures are described in detail hereafter, in the Chapters 4 and 5, respectively – Sections 4.1 and 5.1. The electronic transport features of the SMEs integrating the micro- and the nanoscale devices are also discussed in Sections 4.2 and 5.2. In addition, device applications derived from both the device platforms (Fig. 1.1) were suggested and demonstrated as well. Specifically, a water-gated transistor based on SMEs is presented in Section 4.3, a single-charge device is unveiled in Section 5.3, and a nanomembrane-based pressure sensor is proposed in Section 5.4. The main finds are debated at the end of each chapter, in the format of conclusions (Sections 4.4 and 5.5), and summarized in the Chapter 6.
Motivation (state of the art)
In the past decades, SMEs have become promising candidates in the fields of MolEl and OrEl, with economic interest driven by the display and sustainable energy industries. As the essential components of molecular-based devices,
SMEs have also manifested their importance in understanding various fundamental properties of the condensed matter, from the thermo-mechanical features (Fig. 2.1a,c) to the (opto)electronic (Fig. 2.1b) and/or spin-dependent ones (Fig. 2.1d). Even more significantly, ensemble-MolEl is expected to provide massive device-scalability in between the micro- and the nanoscale due to the possibility to integrate the mentioned complex functions into circuits at the same time (Fig. 2.1e), once just a single-molecule is itself an entire functional system. Apparently, all types of ensemble-molecular devices depend on addressing molecular ensembles through electrode-gaps. Such a task frequently uses the evaporated top electrode approach, which causes the metal atoms diffusion into the SMEs due to the fragile molecular ensemble nature. This issue has restricted the development of integrated ensemble-molecular devices since the MolEl-concept was introduced for almost half a century.[5] Driven by such motivation, the present work aims to fill the gap between the OrEl and the MolEl by investigating the micro- and the nanoscale electronic transport in SMEs connected by integrated transport junctions.
Figure 2.1: Rich physics accessed through measurements of mechanics (a), optical (b), thermoelectric (c), and quantum mechanical spin-dependent phenomena (d) in single-molecule transport junctions. Panels (a-d) adapted from reference [55]. (e) Illustration of an integrated ensemble-molecule functional device for MolEl applications (adapted from reference [5]).
applying a voltage bias, stimulates the molecules to transport charge carriers.[2,5,10] The transport phenomena usually occur into the nanoscale, with transport distances (tMolEl) in the
range of few nanometers (tMolEl < ~10 nm).[5,10,56] As a general quest, MolEl aims to tailor
the behavior of an electronic junction by starting from the most basic, compact building block – the molecule.[5]
OrEl, on the other hand, regards electronic devices commonly composed of polymers or conjugated molecules which form thicker molecular ensembles (> 50 nm) referred as organic thin-films.[15,57] In these cases, the conductive pathways (tOrEl) usually exceed the ensemble
thicknesses, and the electronic transport is characterized by carrier’s longer-time interactions resulting in thermo-activated dependences of the current densities.[5,57] In this sense, for the same molecular structures, the transport mechanisms in MolEl junctions (fast, activationless, carrier tunneling) are usually dissimilar from the conduction ones in OrEl devices (slow, thermo-activated, carrier scattering).[5] The separation between both comprises an unclear transition which happens at a certain length.[5,9,10,33,56] Surprisingly, the electrostatic aspects of both the situations, such as the energy alignment between molecule(s) and electrodes, are similar for the two fields of MolEl and OrEl.[5,12]
Between similarities and divergences, a consensus that molecular ensembles would fill the lack between MolEl and OrEl – leading the molecular functional building block from the nano- to the microscale (bottom-up approaches), and bringing the microelectronic integration to the MolEl (top-down technologies) – is emerging in the literature.[5,10,16] In such a complex scenario, the Chapter 3 sheds some light on the theoretical background needed to investigate the electronic transport in molecular ensembles. Section 3.1 regards the main charge transport mechanisms expected to rule the molecular ensemble conduction. In Section 3.2, it is exhibited a thorough discussion on reliably connecting electrodes and molecular ensembles each other. Finally, Section 3.3 demonstrates one of the few device architectures that allows one to perform MolEl investigations integrated with the standard microfabrication processes – the nanomembrane-based transport junction.
Electronic transport mechanisms
The established triumph of the microelectronics is related to recognize approximate relations.[57,58] This method was found advantageous also for more complex materials, where fundamental investigations on charge transport were needed, such as DNA molecules[28,30,33] and SMEs.[9,10,16,56] A portion of the MolEl community, though, still insists on detailed quantum-mechanical approaches – due to the high sensitivity of the net transport to atomic details. The quantum-mechanical treatments have demonstrated themselves to be as good as the precision of the input parameters, which are arduous to verify during the experiments due to the uncertainty principle.[5] The standard gold−thiol bond, for example, involves a large number of variants on the precise S−Au interaction.[12] Such variants may depend on both the adsorption conditions and the type of molecular tails.[5,12] Hence, any accurate computation becomes very specific to structural assumptions, difficulting the comparative analysis between different measurement platforms, and hindering the device optimization according to the envisioned transport feature or functionality.[12] Additionally, computational generalizations to gain predictive power – from a specific to a generic case, as a bottom-up approach – is often a laborious task in MolEl.[2,5] Thus, without detracting from the quantum-mechanical modeling, another portion of the MolEl community agrees with the well-known power of the so-called “black-box” relations – commonly associated with generic fitting approaches, which are based on the following motivations, according to Vilan, A. et al.:[5]
(i) Identification of the electronic transport mechanism and comparison with both computational simulations and theory, allowing predictions;
(ii) Evaluation of the electronic response and reproducibility of molecular junctions achieved in distinct platforms;
(iii) Insight on how specific molecular modifications change the junction electronic features.
In the brief scenario exhibited, one may realize the “black-box” relations are rooted in the quantum-mechanics but blind to extensive details. In this section, these relations are shown to imply figures of merits which have been demonstrated crucial to both, the electronic transport comprehension, and the further development of MolEl functional applications.[9,59–61]
Fig. 3.1.1 exhibits a classification of the electronic transport mechanisms according to their correlation with the interface- or the bulk-properties of generic SMEs. To measure the
Figure 3.1.1: Classification of the conventional charge transport mechanisms in SMEs.
transport characteristics through a SME, one must prepare a testing device by bringing electrodes in contact with the SME. The most common way is architecting a “sandwich” structure where the SME is placed in between two metallic electrodes, forming a metal/SME/metal junction. The simplest “sandwich” architecture involves the use of two electrodes made of same metals to guarantee similar work functions at the electrode/SME interfaces. For the conjugated SMEs, formed by a vast set of molecules, such aspect leads to symmetric junctions with same interfacial energy barriers.[5,10,58] Complementarily, measuring the charge transport often requires temperature-dependence evaluations of the junctions investigated, even if the mechanism is not thermo-activated.[9,58]
Hence, the interface-limited mechanisms include Schottky emission, DT, Fowler-Nordheim (FN) tunneling, and thermionic-field emission. The bulk-limited ones include ohmic conduction, space-charge-limited conduction, Frenkel-Poole emission, and activated hopping. The methods to distinguish these transport mechanisms are demonstrated hereafter in terms of both the schematics energy band diagrams (Figures 3.1.2 – 3.1.5) and the J-E equations for each transport mechanism mentioned in Fig. 3.1.1. In the energy band diagrams of Figures 3.1.2 – 3.1.5, the highest and the lowest occupied molecular orbitals (HOMO and LUMO, respectively) of the SME are schematized, the charge carriers are represented by circles (playing the role of electrons), and the transport mechanism is identified inset. Additionally, activated transport events are represented by red arrows, and coherent (or activationless) mechanisms are denoted by gray arrows in Figures 3.1.2 – 3.1.5.
One of the most often observed conduction mechanism in semiconducting and dielectric structures, especially at a relatively high temperature (more than ~300 K), is the Schottky emission. Such a mechanism is also known as thermionic emission due to the effect of temperature on the electronic transport: if the charge carriers present at the metal electrode can obtain enough energy, provided by thermal activation, they overcome the energy barrier at the
metal/SME interface to compose the conduction across the junction. Fig. 3.1.2a exhibits the schematics energy band diagram for the Schottky emission occurring in a metal/t-thick SME/metal junction. The left-handed metal electrode is under negative bias (-V) with respect to the right-handed metal electrode, which performs the role of substrate in vertical junctions. As t has a magnitude of nanometers, the electrostatic potential may be well approximated by a decreasing straight line along the ensemble thickness (Fig. 3.1.2a). Such an assumption leads to the uniform electric field (E) in the SME. For the Schottky emission, the energy barrier height at the metal/SME interface is lowered due to image forces (Schottky effect), and the current density is given by:[58]
(Eq. 3.1.1)
where J is the current density, A* is the effective Richardson constant, T is the absolute temperature, q is the electronic charge of the carriers, qB is the Schottky barrier height, E is
the electric field across the SME, r is the dielectric constant, 0 is the permittivity in vacuum
conditions, k is the Boltzmann’s constant, m0 is the free electron mass, m* is the effective
electron mass into the SME, and h is the Planck’s constant.
According to the classical physics, when the incident charge carriers have energies lower than a generic potential barrier, they would be reflected. However, if the barrier is thin enough, the quantum mechanics predicts the carrier wave function may penetrate through the referred potential barrier.[62] Hence, for the lower thicknesses (t < 10 nm), and low temperatures (T < 100 K), to prevent charge carrier activation, another transport mechanism have often been observed in SMEs: the DT. Fig. 3.1.2b illustrates, via a schematics band diagram, the direct tunneling event occurring for a metal/t-thick SME/metal transport junction. Tunneling is a coherent charge transport mechanism, which takes place due to non-negligible transmission probabilities, resultant from the quantum confinement of the charge carriers by high but narrow energy barriers. As tunneling is based on transmission functions, the charge carriers exhibit low interaction-times within the molecular energy barriers. As a consequence, during a tunneling event, the carriers preserve the phase of their wave functions – leading to a coherent transport. Hence, tunneling events also have exhibited observable “activationless” features,[9,10] with current densities strongly affected by the transport distance (i.e., the barrier width t) and weakly dependent on T. Analytically, the DT current density can
Figure 3.1.2: Schematics energy band diagrams of Schottky emission (a) and direct tunneling (b) in electronic junctions formed by metal/SME/metal.
be approximated by:[63]
(Eq. 3.1.2)
where is a decay coefficient associated with the barrier height qB, and with the effective
mass m* of the charge carriers. The other terms were defined previously.
The decay coefficient is also referred as the attenuation factor of the coherent transport. It determines how fast J decreases with the increasing of t, performing a fundamental role for the nanoscale electronic transport investigations. This concept may be extrapolated to the activated transport, where the -values are often ~ zero and the mechanisms are found to be diffusive, as discussed in Chapter 5, Section 5.2.
In the presence of higher electric fields, if the semiconducting material withstands, the representative band structures of the SME may bend until the DT energy barriers become trapezoidal. In such scenario, illustrated by the band diagram of Fig. 3.1.3a, the FN tunneling takes place. Notice that such a mechanism may occur for semiconducting bridges thicker than the DT barriers because E assists the tunneling event by lowering the tunnel distance in the junction. One may visualize the field-assistance effect by comparing Fig. 3.1.2b with
Fig. 3.1.3a, observing that the effective tunneling distance, when FN tunneling takes place, is smaller than t. Accordingly, the field-assisted tunneling can be described approximately by:[63]
(Eq. 3.1.3)
where (V) is an attenuation function – rather than a single-value parameter as described for the
DT – dependent on V, the carrier effective mass m*, and the barrier height qB.[63] Namely,
(V) scales with V-1, meaning that the current density attenuation is strongly dependent on the
bias in high E regimes. Consequently, in the Fowler-Nordheim tunneling, the higher the V, the lower the transport distance, and the higher is the tunneling currents.
Between the FN tunneling (Fig. 3.1.3a) and the Schottky emission (Fig. 3.1.2a), another charge transport mechanism is known to take place in SMEs: the thermionic-field emission. Such a mechanism is a composition of activated effects and coherent field-assistance of the charge carriers, as schematized in Fig. 3.1.3b. The current density exhibited by the thermionic-field emission can be expressed by:[58]
(Eq. 3.1.4)
where the two mentioned contributions are easily identified: (i) the thermoactivated due to the temperature T present in the exponential factors, and (ii) the field-assisted related to the electric field present in the second exponential term. Due to the activated stimulus, the thermionic-field emission does not preserve the phase of the charge carrier wave function, as occur for both the direct and the FN tunneling events.
As t increases more than a few of nanometers (t > 10 nm), the charge carriers prolong their stay into the SME bulk during the electronic conduction across the metal/SME/metal junction. In this scenario, the bulk-limited transport mechanisms start to play the major role. Such mechanisms depend strongly on the electronic properties of the SME itself rather than the electrode interfaces. The SME trap energy level becomes a crucial parameter for the conduction limited by the material bulk. Besides the trap energy level, other electronic properties of the SMEs may be evaluated by the investigations on the bulk-limited transport: the trap spacing, the trap density, the electronic mobilities, the density of states, and so on.[9,10,16,57,58]
Figure 3.1.3: Schematics energy band diagrams of Fowler-Nordheim tunneling (a) and thermionic-field emission (b) in electronic junctions formed by metal/SME/metal.
The simplest bulk-limited transport mechanism is the ohmic conduction. As illustrated in Fig. 3.1.4a, ohmic currents are caused by mobile charge carriers (electrons, in the case presented here) flowing in the LUMO of the SME bulk, resulting in a linear relationship between the current density and the electric field. The mobile electrons may be excited to the material LUMO from an impurity level or even from the HOMO of the SME, due to thermal activation. The ohmic current density, then, can be expressed by the following relation:[58]
(Eq. 3.1.5)
where is the electrical conductivity, n is the number of carriers in the ensemble LUMO, is the carrier mobility, NLUMO is the density of states in the LUMO level, and the other terms were
defined previously. It is worth mentioning that Eq. 3.1.5 refers to the case in which the charge carriers are excited from energy levels near to the Fermi level – approximately (EHOMO – ELUMO)/2 – to the ensemble LUMO. If the ohmic current is generated by thermal
activation of carriers from impurity levels, such energy values are referred as an activation energy Ea < (EHOMO – ELUMO)/2.[57] Generally, for SMEs, the ohmic currents are reported to be
very small due to their large bandgap of > 1~2 eV.[57] But, although small, the overall ohmic conduction is never negligible, playing a fundamental role when no contributions of other transport processes are achieved [57,58] – i.e., at very low electric fields applied to the SMEs.
For the situation in which the carrier thermal excitation from trap sites occurs, and high electric fields are applied (if the molecular structure of the ensemble withstands), the conduction limited by space-charges within the SME bridge can be observed.[57] The space-charge-limited conduction can be visualized by the energy band diagram depicted in Fig. 3.1.4b. Notice that two situations may be evidenced: (i) the charge carriers during the formation of the space-charge region (SCR), and (ii) the space-charge carriers flowing through the SCR.
The space-charge-limited currents (SCLCs) are due to the high current density reached in the SME at high E conditions. In this scenario, the excess of charge carriers in the junction influences the pathway of the others and their average transit time becomes equal to the SME dielectric relaxation time.[57,64] Consequently, a space-charge region is formed within the SME, resulting in an energy barrier dependent on the SCR length, which causes a quadratic dependence of J on E, according to the relation:
𝐽 =9 8[
𝜇𝜀𝜃
𝐿 ] 𝐸2 (𝐸𝑞. 3.1.6)
where is the ratio between free and total carrier densities, and L is the separation between the metal electrodes in Fig. 3.1.4b.
As mentioned above, Fig. 3.1.4b illustrates two conditions of the SCLC in SMEs. The situation (i) refers to the traps-filled limit, where the SCR is being formed due to the filling of the trap states in the SME LUMO level by the excess of injected carriers. Accordingly, is smaller than 1 in Eq. 3.1.6 for the traps-filled limit.[64] The further increase of E may also increase the density of free carriers. The current injected in this circumstance (ii) causes the Fermi level of the molecular ensemble to move up, above the trapping levels. Consequently, in the region (ii) the overall transport becomes entirely controlled by the space-charge, which limits the subsequent injection of free carriers in the SME. The situation (ii) is called “Child’s regime” because of the current density follows the Child’s law ( = 1 in Eq. 3.1.6).[64] Additionally, it is worth mentioning that the space-charge-limited conduction is commonly observed in high E, but succeeding the observation of ohmic currents in low E. The transition
Figure 3.1.4: Schematics energy band diagrams of ohmic conduction (a) and space-charge-limited conduction (b) in electronic junctions formed by metal/SME/metal. The SCR is also illustrated in panel (b).
electric field between the ohmic conduction and the situation (i) – the traps-filled regime – is given by:[57]
𝐸(𝑖) =8 9
𝑞𝑛0𝐿
𝜀𝑟 𝜃 (𝐸𝑞. 3.1.7)
where n0 is the concentration of free charge carriers in thermal equilibrium.
Finally, the transition electric field between the situations (i) and (ii) – between the traps-filled and the Child’s regimes – is also known to be estimated by:[57]
𝐸(𝑖𝑖)= 𝑞𝑁𝐼𝐿
2𝜀𝑟 (𝐸𝑞. 3.1.8)
where NI is the impurity density. The other terms in Equations 3.1.7 and 3.1.8 are defined
previously. In summary, from ohmic and space-charge-limited currents, one may access different electronic properties of molecular ensembles, such as Ea, n0, E(i), E(ii), , NI, r, and .
performed in Chapter 4, by bringing the electronic transport approach to investigate the conduction features of microscale devices based on SMEs.
Another charge transport mechanism that is limited by the bulk of the SME is the Frenkel-Poole emission, in which the thermal excitation of entrapped charge carriers launches them into the LUMO level of the ensemble. Hence, Frenkel-Poole emission can sometimes be treated as “an internal Schottky emission.”[58] Considering an electron localized into a trapping center of the SME bandgap, when an external electric field is applied across the SME bulk, the Coulomb potential of the entrapped electron can be reduced, as represented by the bends of the LUMO potential wells in Fig. 3.1.5a. Such a decrease in potential energy may increase the probability of thermal-activation of the electron out of the trapping center,[62] into the LUMO level of the SME. The schematics of Frenkel-Poole emission, regarding the metal/SME/metal band diagram, is also illustrated in Fig. 3.1.5a. Notice that the energy barrier heights are related to the deepest trapping centers into the LUMO level of the SME, namely qT.
Since a Coulomb attraction occurs between carriers and trapping centers, the Frenkel-Poole emission is expected to exhibit a current density given by:[58]
(Eq. 3.1.9)
where T is the trapping potential. Since Frenkel-Poole emission depends on both the
thermal-activation and the applied electric field, high temperatures and high electric fields are favorable to observe such a mechanism taking place in molecular ensembles.[16]
Finally, one may question what happens if the trapping sites separation into the SME bandgap becomes smaller, in the order of few nanometers, for instance. In this situation, the hopping conduction takes place, as schematized in Fig. 3.1.5b. Such a bulk-limited transport mechanism occurs due to the tunneling effect of entrapped charge carriers that “hop” from one trapping center to the other in the SME. According to the hopping theory, the current density is expected to follow the equation:[16,58]
J = q NLUMO exp [-/ kT ] (Eq. 3.1.10)
where is the mean hopping distance, and is the frequency of electron thermal vibration at the trapping centers. The temperature-dependence of J arises from the long interaction time of
Figure 3.1.5: Schematics energy band diagrams of Frenkel-Poole emission (a) and space-hopping conduction (b) in electronic junctions formed by metal/SME/metal.
charge carriers within the molecular bridge, due to the long transport distances achieved in the sequential process.[10,56] Such core aspects lead to SME dielectric relaxation during the hopping conduction, and consequently to the phase decoherence of the electronic wave function.[9,10,33,56] Some authors attribute hopping conduction to the sequential tunneling due to the vibronic coupling of charge carriers with the vibrational modes of the molecular ensembles.[56] The field-dependence of hopping is hidden into the potential well T, which
may exhibit a small reduction according to the electric field increment due to the coulombic effects.[58] Notice that the Frenkel-Poole emission corresponds to the bulk-thermionic effect, and the hopping conduction refers to the thermoactivated sequential tunneling. Therefore, a transition from one mechanism to the other, respectively, may occur as a function of the electric field – as reported by Bof Bufon et al. for copper phthalocyanine molecular ensembles connected by Au electrodes.[16]
Connecting molecular ensembles
As commented in Chapter 1, one of the biggest issues in MolEl is the challenge of connecting molecules (nanometric objects) to the outside environment. The fragility and reactivity of the organic molecules constitute inherent difficulties, while the electrode diffusion through the molecular structures may complicate the scenario.[5] Also, the electrode’s surface roughness becomes crucial to ensure an effective contact, faced to grain-boundary dimensions similar to, or even larger than, the molecular objects being probed.[10] Although apparently technical, such an issue is of fundamental importance that, according to Vilan et al., 2017, is “touching the heart of bottom-up nanotechnology”.[5]
To bring electrodes in effective contact with molecules often demands a “sandwich” configuration, as shown in Fig. 3.2.1a. Considering the molecular ensembles, such an electrode-arrangement has the merit of using the molecules as a ruler[5] that orders the spacing between the electrodes – i.e., the electronic transport length. The “sandwich” approaches are made in two steps: first, a molecular ensemble is placed on a bottom electrode; next, a top contact is formed on the ensemble. Original works on MolEl used such a “sandwich” architecture to connect self-assembly monolayers.[65,66] The major difficulty displayed by the “sandwich” approach is the extent to which the molecules are affected, upon the top contact placing/formation. The “trench” is an alternative method where both electrodes can be patterned at a first stage, and just later the molecular ensembles are adsorbed simultaneously to the electrodes.[67,68] For this approach (sketched in Fig. 3.2.1b), the trench between the electrodes is commonly prepared by advanced techniques of lithography, or even by electromigration.[69,70] If the trench is thought to be vertical, a thin insulating layer can define the overall transport distance.[67,68] The “trench” approach is key for gating or concurrent spectroscopy, due to its open geometry.[71,72] In contrast, the insulating layer also adds a major problem of leakage current, if the tested ensemble does not conduct considerably better than the supporting insulator – as observed for the SMEs.[5]
An alternative route to connect molecular ensembles preventing structural damages is using temporal contacts, which are in principle (i) less invasive to the molecules, and (ii) more comfortable to use for multiple repeated measurements than the permanent electrodes. One of the temporal contacting techniques is based on scanning probe systems that allow the formation of junctions with intermediate characteristics, between single-molecules and molecular
Figure 3.2.1: Schematic prototypes of ensemble-molecular junctions. (a) “Sandwich” method to bring electrodes in contact with molecules (wiggly lines) – which are anchored via a binding group (blue triangle) to the substrate and have a terminal group (pink circle), pointing toward the top contact (yellowish). (b) Corresponding materials illustrated in (a), but in a “trench” device configuration. (c) Illustration of the contacting method based on metallic liquids (Hg, in the case). Panels (a) and (b) were adapted from reference [5]. Panel (c) was adapted from reference [4].
ensembles.[73–75] However, apparently, scanning probe approaches for MolEl are aimed at lab use only, not for integrated devices. Another method involves the use of metallic liquids, as illustrated in Fig. 3.2.1c.[5,17] Specifically, the semi-noble character and the high surface tension of Hg made it an ideal candidate for contacting molecular ensembles. However, the known Hg-vapor amalgamation strongly limits its usefulness for contacting molecules attached to the popular substrates (Au, Ag, II-VI semiconductors).[5] The Hg reactivity issue can be solved by adsorbing a thiol-linked self-assembly monolayer on the Hg-drop surface, but the electronic transport properties of the system of interest cannot be determined directly.[76–78] The technological improvements have led to contact self-assembly monolayers on noble metals by using eutectic GaIn (EGaIn).[17] Liquid InGa alloys exhibit both lower vapor pressure and lower reactivity (toward most of the substrates) than Hg, but they are covered by an imprecise Ga−oxide layer, which complicates the quantitative transport modeling.[79] Overall, notice that the mentioned liquid metal contacting approaches go against the miniaturization, the fabrication of nanoscale integrated devices, and the possibility of performing temperature-dependent measurements in ensemble-molecular junctions.[5] Additionally, the “lift-off, float-on” approach can also be used to transfer a solution-lifted electrode to the top of a “sandwich” structure,[80] or even a magnetic field can be used to bend a narrow electrode, bringing it into contact with an opposing electrode covered by a molecular ensemble.[81] Anyway, temporal approaches to connect molecules at the nanoscale are still restricted to the research laboratories, and are incompatible with the most device fabrication processes.
Figure 3.2.2: Illustration of a nanoscale transport junction based on the deposited top electrode. The deposition direction is indicated on the left side. The transport distance t is indicated at the right side. Combined characteristics of the organic nature of molecular ensembles – such as internal and topographical irregularities, softness, vacancies, and structural defects – may lead to issues (i-v) commonly found up for junctions in which the upper electrode is deposited on molecules. Such issues may result in severe artifacts for measurements of electronic transport in molecules: (i) molecular damage due to degradation; (ii) short circuit due to direct contact between upper and lower electrodes; (iii) structural damage to the molecular ensemble due to penetration of deposited material or structural rupture; (iv) shortening of the transport channel; (v) migration of agglomerates (clusters or nanoparticles) of electrode material into the junction.
In fact, the device fabrication reliability arises from the permanent “sandwich” approaches, most often made by evaporation of metallic films. Even though crucial contributions to the MolEl were demonstrated for molecular junctions based on evaporated top contacts,[65,66,82,83] this approach was severely criticized due to the damage caused to the molecular ensembles.[84–86] For example, sample heating (thermal source) and/or electron irradiation during physical deposition can also cause damage to the monolayer – as molecular fractures and deprotonation, leading to cross-linking or unsaturation, as illustrated in Fig. 3.2.2, issue (i). Other molecular issues caused by the top electrode evaporation are identified in Fig. 3.2.2a (ii-v) and described in the figure caption. It is worth mentioning that the use of nanopores[82] mitigates the damages suffered by the molecular ensembles during fabrication, but the yield of working devices is rather low, hampering the identification of “genuine” devices.[5] Another approach to reduce the evaporation issues is performing indirect evaporation. For this, the sample is back-faced to the source, and the evaporated atoms reach the substrate by scattering.[87] Buffer layers that evaporate when the sample is heated to room temperature[88] were also used to prevent molecular damages during evaporation. In such
The nanomembrane-based transport junction
Nanomembranes (NMs) are freestanding, surface-like structures[93–95] with a reduced thickness (1 – 100 nm) in comparison with their lateral dimensions (typically in the m-range).[94] NMs essentially differ from thin-films due to their occurrence in self-sustained, isolated forms in as-fabricated devices, or at some stage of their manufacturing. Because of their versatility and unique mechanical properties, NMs offer many possibilities that may be of interest for several applications, expanding the frontiers of nanoscience and nanotechnology.[95–97] Reports on NMs date back to approximately thirty-five years ago with experimental demonstrations on cadmium-based nanocrystals[98] and fullerenes.[99] Further investigations on such materials rapidly evolved to their processing in devices, where nanowires and carbon nanotubes were used in attempting to establish robust electrical contacts to individual nano-objects.[100] However, these practical applications based on connecting one-dimensional structures to the system of interest faced, over the years, tremendous engineering challenges to be addressed in high-yield, scalable fabrication processes, and manufacturing.[94] In such a complex scenario, NMs arose as structures with great potential to circumvent the referred limitations due to their intrinsic compatibility with the well-established “top-down” fabrication technologies employed in the semiconductor industry.[10,91,95,97,101] Thereby, NMs are found integrating a myriad of functional device components, including compliant substrates to accommodate quantum dots and nanoparticles,[96] stretchable electronic elements,[102–104] solar cells,[105,106] field-effect transistors,[97,107,108] single-photon emitters,[109] and so on.[91,97,101] Fig. 3.3.1 exhibits some of the most relevant works using NM-based devices/components that were developed in the past half-decade.[95,110–113]
NMs have also exhibited their excellence in the fields of nano- and molecular electronics, where their main appeal arises from the possibility of bringing to the electronic devices properties which cannot be addressed with any other bulk, thin-film, or nanoscale material.[95] Flexible and stretchable electronics have benefited from advanced mechanical layouts and 3D nanoarchitectures based on NMs.[114–119] The claim for NMs in bioelectronics applications has also increased due to the possibility to build bio-integrated circuits on NM-based substrates which laminate non-invasively onto the organic fabrics and monitor vital signs.[120–122]
Figure 3.3.1: Relevant works on NMs that were highlighted on the cover page of high impact scientific papers in the past half-decade. (a) Yan, C. et al., 2013.[110] (b) Sharma, R et al., 2014.[111] (c) Xi, W. et al., 2014.[112] (d) Wang, X. et al., 2016.[95] (e) Tian, Z. et al., 2017.[113]
For nanoelectronics, state-of-art applications using NMs are straightly tied up to the molecular electronics research and technology fields.[10,16,89–92,97,101,111] Such structures have been used to promote reliable electrical contacts to molecular ensembles or hybrid organic/inorganic materials, forming NM-based large-area devices – e.g., transport junctions,[10] molecular diodes,[89] and ultra-compact capacitors.[101] Fig. 3.3.2a illustrates the device structure before and after the formation of the NM-based junction. For this method, a silicon oxide structure (finger) is patterned on a substrate made of the same material. Such a finger structure accommodates the junction bottom-electrode (named finger electrode). A strained layer, then, is deposited on a patterned sacrificial layer, strategically positioned surrounding the finger electrode in the sample plane. The sacrificial layer just has the task of hold the strained nanomembrane on the substrate while the NM is not ready to roll-up. Finally, the molecular ensemble is placed on the top of the finger electrode, and the sample is ready to frame the so-called NM-based junction. Such patterned sample is all fabricated by the standard microfabrication techniques. The methods and materials are detailed in Chapter 5, Section 5.1. The overall NM-based structure, before the roll-up stage, is illustrated on top of Fig. 3.3.2a.
The selective removal of the sacrificial layer, indicated in the top of the Fig. 3.3.2a (red layer), promotes the relaxation of the strain stored in the nanomembrane, which stands itself over the sample plane, becoming freestanding.[123] The strain relaxation makes the NM bends over, assuming a cylindrical shape with a typical diameter of ~8 μm. Fig. 3.3.2b exhibits the scanning electron microscopy (SEM) image of the NM-based device.
Figure 3.3.2: (a) Schematics integrated MolEl-device before (top) and after (bottom) the roll-up process. The controlled strain relaxation curls roll-up the metallic NM (Au-coated) into a cylindrical shape, establishing the junction top electrode. (b) SEM image of the NM-based device. (c) Lateral view of the Au/SME/Au transport junction. The conductive channel is ruled by the ensemble thickness (t). Figure adapted from reference [10].
The rolled-up NM contacts the molecular ensemble from the top, providing a soft, robust, and self-adjusted electrode, forming molecular junctions integrated with the standard microfabrication processes.[10,16] The NM-based junction is illustrated in Fig. 3.3.2c. The mentioned characteristics make the NM-based device a suitable platform to consistently characterize thin and ultrathin molecular ensembles, allowing stable electrical measurements as a function of the temperature, the applied electric field, the mechanical stress, and the ensemble thickness – as performed hereafter (Chapter 5) for physisorbed molecular ensembles.
In this chapter, the influence of the electrode configuration on the electrical properties of SMEs is systematically investigated from the unexplored charge transport perspective. For this purpose, the prototype organic semiconductor CuPc is employed to fabricate two-terminal, planar devices designed to minimize interfacial contact effects. As the primary outcome, it is demonstrated that space-charge limited conduction dominates the SME response when bottom electrodes connect the samples.[57] For SMEs connected from the top, the electrical conduction is ohmic. The I-E characteristics, for different temperatures and ensemble thicknesses, and the finite-element calculations provide clear evidences that space-charge limited conduction arises from traps at the ensemble/substrate interface. The experimental findings show that the electrode configuration has a central role in the electrical response of SMEs and the contact position must be selected according to the device application. For example, top-electrode configuration appears more suited to detect phenomena occurring on top of the device, e.g., in gas or humidity sensors. Samples connected by bottom electrodes, on the other hand, are convenient for applications involving substrate modifications (e.g., by self-assembly monolayer) or when environmental effects (e.g., oxygen and humidity) need to be avoided. By considering such a fact, the bottom-electrode platform was employed to fabricate a water-gated, organic field-effect transistor (OFET), capable of detecting the bio-specific interaction between the glutathione and the glutathione S-transferase – a pair of biomolecules correlated with neurodegenerative diseases.[124] The fabrication, operation and biosensing application of the water-gated OFET are described as follows.
Experimental details
To investigate the role of device architecture on the electrical properties of organic, thin molecular ensembles, devices in two different configurations were fabricated, the top-contact (TC) and the bottom-contact (BC) architectures, as shown in Figures 4.1.1a and 4.1.1b, respectively. Both the architectures were assembled onto glass slides using microfabrication. An oxygen plasma treatment powered by 150-200 W per 15 minutes was employed to remove adventitious organic contaminant on the glass substrate surfaces. As electrodes, 50 nm thick Au films were deposited by e-beam evaporation in high vacuum (5 x 10-6 Torr) at a rate of 0.5 Å/s. For the BC devices, fabricated using optical lithography, an adhesion layer of Cr was deposited prior to the Au electrode, under similar deposition conditions. For the TC devices, the top-electrodes were deposited onto the SMEs with geometry transferred by a shadow mask. For both architectures, CuPc (Alfa Aesar, powder, 576.08 g/mol) was sublimated at a rate of 2.0 Å/s and pressure of 2 10-6 Torr at two different thicknesses, t = 50 nm and 500 nm. The substrates were kept at room temperature during the CuPc deposition.
Figure 4.1.1: Device layout in (a) TC and (b) BC configurations (figures adapted from reference [57]). W, L, and t indicate, respectively, the electrode width, the channel length and the thickness of the CuPc ensemble. (c) Picture of the typical device, with the CuPc molecule depicted inset. (d) Atomic force microscopy (AFM) image of the CuPc ensemble. (e) The topographic profile obtained from the straight line in panel (d).
A picture of the typical device is shown in Fig. 4.1.1c. Specifically, such a glass slide is held by a hand, and contains five as-fabricated, BC devices. The blue layer covering the interdigitated bottom-electrodes corresponds to a 50 nm thick CuPc molecular ensemble. The CuPc molecule is depicted inset. Notice the π-conjugated structure of CuPc, which promotes the electron delocalization and leads to its well-known semiconducting properties.[125] The topography of the CuPc molecular ensembles was evaluated by AFM. Fig. 4.1.1d exhibits the AFM image obtained from a sample region where a gentle scratch was done. The arrow indicates the glass substrate surface. The bright regions refer to the CuPc molecular ensemble, whose topography can be observed at the left side of Fig. 4.1.1d. The straight line “1” delimits the topographic profile exhibited in Fig. 4.1.1e. The effective thickness is measured as
The role of device architecture on the electronic transport
This section presents a systematic analysis of the electrode configuration influence on the electrical properties of planar, two-terminal devices based on SMEs, as depicted in Fig. 4.1.1. The electronic differences found by comparing the top and the bottom contact architectures are presented and investigated. Top-contact configurations have a linear I-E (current vs. electric field) behavior, while the bottom-electrode devices display a transition from the ohmic regime to the space-charge-limited one. The transition is temperature- and thickness-dependent. Finite-element calculations show that the current flows through the semiconducting bulk when the SME is connected using top electrodes. On the other hand, the bottom-contact configuration allows most of the current to flow near the substrate interface. In this scenario, the current probes interfacial states resulting in a space-charge conduction regime. These results shed some light on the so-called “contact effects” commonly observed in organic thin-film transistors. The findings presented here have implications for both the understanding of the charge transport in SME-based films and the design of organic semiconductor devices. The results of this work was published[57] in the journal Organic Electronics (2017), entitled “The
role of the electrode configuration on the electrical properties of small-molecule semiconductor thin-films”, by Leandro Merces, Rafael F. de Oliveira, Henrique L. Gomes, and Carlos C. Bof
