Charge Transport Dilemma of Solution-Processed Nanomaterials

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Charge Transport Dilemma of SolutionProcessed Assemblies of Nanoparticles Ji-Young Kim, and Nicholas A. Kotov Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm402675k • Publication Date (Web): 24 Sep 2013 Downloaded from http://pubs.acs.org on September 29, 2013

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Chemistry of Materials

Charge Transport Dilemma of Solution-Processed Assemblies of Nanoparticles Ji-Young Kim and Nicholas A. Kotov*

Department of Chemical Engineering, Department of Materials Science and Engineering, Department of Biomedical Engineering, University of Michigan University of Michigan, Ann Arbor, MI, 48109 E-mail: [email protected]

ABSTRACT: A large variety of nanoparticles and similar nanocolloids were synthesized during the last 25 years and can be considered now as “building blocks” for a variety of materials. Bottom-up solution processing of devices emerged a promising direction of their technological applications because this method can (a) utilize intrinsic ability of nanocolloids to self-organize; (b) reduce high energy and equipment cost of device manufacturing, and (c) impart new functionalities to nanoscale electronics. However the technological impact of solution processable semiconductor materials -- although potentially considerable -- was so far limited because of the long-standing dilemma between the need for effective colloidal stabilization and effective charge transport. Surfactants and other organic materials being used to synthesize and/or disperse nanocolloids introduce a barrier for charge transport between the particles. In this review, we look into the latest progress in the solution processable devices and methods to produce electrically conductive thin films from nanoparticles and other nanocolloids. We are specifically interested in the understanding of the prospects of self-assembly to facilitate charge transport and nanoscale connectivity during solution processing. The updated theoretical description of charge transport in nanoparticle solids and similar nanomaterials is also given. It includes consideration of the key mechanisms such as tunneling and cotunneling, as well as key electrical parameters characterizing transport of electrons through the surfactant-related barriers, such as coupling energy and Coulombic charging energy. Manifestations of these mechanisms in different electronic materials made from nanoparticles, nanowires, nanotubes, and nanosheets, their relative advantages and disadvantages are also discussed in details. We conclude the topic with a brief description of new opportunities and approaches to improve charge transport in solution processed materials for electronics applications.

Keywords: nanoparticles, self-assembly, charge transport, electronic devices, flexible electronics, tunneling, epitaxial attachment, conductive thin films, solution processing

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1. Introduction Unique properties of nanomaterials, such as size-dependent bandgap, ballistic electronic transport, high surface area, depressed melting temperature, and others1,2,3,4 are promising for the resolution of considerable challenges facing today’s electronics industry. These challenges include increasing operating frequency, information density, charge storage capacity, and widening of the wavelength window, as well as decreasing heat generation, power consumption, and cost. To date, commercial fabrication of electronic devices has been accomplished primarily by “top-down” approaches dominated by classical photolithography and complemented by additional nanofabrication tools including e-beam lithography, 5 , 6 , 7 , 8 electrodeposition,

9

nanoimprint lithography, 10 nano-stencil technique,11 and more.12 Despite the rapid advancement of “top-down” techniques, the high cost of multi-step lithographic processes and availability of the materials suitable for them are presenting ever increasing challenges. Widespread use of the nanofabrication techniques mentioned has been further impeded by slow speed and scalability issues as well.

On the other hand, matured syntheses of nanoparticles (NPs) and other

nanocolloids have allowed for the production of a diverse array of nanomaterials in dispersions in a variety of solvents. These dispersions offer highly crystalline semiconductor/metal materials, exceptional degree of structural control, sufficient batch-to-batch reproducibility, and promise of reduced cost. Consequently, “bottom-up” methods of manufacturing have been emerging as viable pathways to certain electronic devices. While they still lack the some capabilities to be considered a silver bullet for solving many of the technological problems applicable to all challenges of today’s electronics, for instance they fall short in terms of discretionary geometric patterns, they have enabled the replacement of expensive high-vacuum deposition steps with


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room temperature solution processing and use of self-organization nanoscale phenomena as an alternative to insufficient spatial resolution of photolithography. The idea of constructing nanoscale devices and machines using small nanometer-sized entities as the building blocks was first formally put forward in 1959 by R. P. Feynman,13 and further popularized by K.E. Drexler during the mid-1980s. 14 However, there were many fundamental problems associated with assembly of such devices. Some of these problems are related to scalability, i.e. the possibility of making a specific type of device reproducibly, while other problems are related to fundamental aspects of the materials used in these devices. The scalability problems belong not only in the domain of process optimization.

Careful

consideration of the seemingly simple task to make the same object a few nanometers in size and reasonable complexity uncovers an abundance of academic challenges which we are just starting to appreciate.

As such, the scalability issues for assembled nanostructures are related to the

intrinsic variability of nanoscale building blocks, thermodynamic limits of self-assembled devices, and the changes in scaling laws when one attempts integration of nano- and microtechnologies.1 Although we shall touch on some scalability topics in this work, the main focus will be on one specific problem among the many materials’ science challenges of bottomup assembled devices. This problem emerges when constructing a consolidated structure from integrants of nanoscale dimensions, using solution-processing methods, with the intention of using this structure in electronics. A fundamental dilemma then arises; on one hand, nanoscale building blocks -- exemplified in many cases by NPs -- must be dispersible in a solvent; therefore, they must carry a protective coating is typically made of surfactants or other organic materials that make NP compatible with a particular organic or aqueous media. The surfactant coating is also necessary to make possible the observation of the size quantization effects and, in



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many cases, to obtain high crystallinity of NPs. On the other hand, this coating must disappear before the nanocolloids assemble into a continuous structure that must have some electronic conductivity to enable the use of the structure as an electronic material.

The problem is that

organic materials coating NP interfere with charge transfer properties of semiconductor or metal colloids and there are no organic conductors that have sufficient mobility along the chain of carbon atoms to compete with that in inorganic materials. We selected this dilemma of bottom-up assembled nanoscale systems because (a) there was a large amount of activity in the area of solution-processable devices, 15,16 (b) there had been no systematic considerations of this problem, and (c) its resolution could have a broad impact including applications in optoelectronics, 17,18,19,20,21,22 chemical/biological/optical sensors, 23,24 as well as energy harvesting, conversion, and storage. 25

2. Thin Films from Nanoscale Dispersions In order to understand better the nanoscale connectivity problems in solution-processable devices, we initially needed to look at the nature of the processes involved in transition of NPs and other nanocolloids from dispersion state to solid state. Current device manufacturing is based on essentially two-dimensional (2D) technology of semiconducting thin films. Methods of their preparation, overall quality, reproducibility, electrical, and sometimes optical properties determine the functions of the devices and their competitiveness of their performance. In this section we shall survey the methods of transforming dispersed nanocolloids into thin film coatings on solid substrates and methods for controlling their nanoscale structure.


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2.1. Nanoparticles Electronic devices could be made from nanoparticle films of different organization (glassy, ordered, liquid crystalline, etc.) as long as they allow the transport of charge in the plane parallel to the substrate. Spin-coating 26 , 27 , dip-coating 28 , spray-coating 29 and drop-casting 24,30,31,32,33,34

of colloidal solutions of NPs has often been used as manufacturing methods for NP

films that typically show short-range ordering. Judicious selection of solvents and modification of the substrate surfaces allowed formation of uniform spin-cast films. Langmuir−Blodgett (LB),35,36,37,38 layer-by-layer deposition (LBL), 39,40,41,42,43 and inkjet printing 44 have been also employed to prepare NP films with short- range ordering on virtually any kind of substrate. Ordered three dimensionally (3D) organized NP solids, also known as superlattices, facilitate practical applications due to their uniformity of material properties throughout the longrange structure, while LBL, ink-jet, and other composite coatings display favorable mechanical properties. 45,46 The ability to mix and match different NPs and a large variety of superlattice patterns offer a variety of novel materials with unique chemical composition and maximum packing density, which is essential for charge transport. Due to such extra degrees of freedom to engineer materials as compared to atomic solids, NP superstructures are being intensively studied by many scientists.47, 48,49,50,51,52,53,54 Nearly monodispersed NPs can self-assemble into long-range ordered superlattices over distances exceeding hundreds of micrometers (Figure 1A) upon slow evaporation of carrier solvent 55,47 or destabilization of the colloidal solution.56,57,58,59 In the simplest case NPs form face centered cubic (fcc) lattice;47 a large variety of other packing patterns are possible for geometrically anisotropic NPs and binary NP superlattices (BNSLs). Shevchenko et al.



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demonstrated formation of more than 15 different BNSL structures, using combinations of semiconducting, metallic, and magnetic nanoparticle building blocks60 (Figure 1B). Unusual superlattices indicate large variety of self-organized structures that could be potentially observed for such NP systems. Talapin et al61 also have observed non-close-packed simple-hexagonal superlattice of nearly spherical PbS, PbSe, and γ-Fe2O3 NPs, which goes beyond the prediction of hard sphere models. Taking into account dipole-dipole interaction, they demonstrate the complexity of NP phase diagram and predict anti-parallel ordering in the lattice. Strongly enhanced p-type electronic conductivity of multicomponent BNSLs has been reported by Urban et al., who demonstrated that nanocrystals could behave as dopants in NP assemblies. 62

Recently, a general method of growing centimeter-scale, uniform membranes of BNSLs that

can readily be transferred to arbitrary substrates has been reported by Dong et al. 63 Solution-processed NPs are generally synthesized with a coating from surfactants also known as capping agents/ligand or stabilizer. These surfactants often form, bulky and insulating shells on each NP. The presence of the surfactant layer results in interparticle spacing and an insulating layer in the NPs network, thus seriously impeding charge transport through the film. As the very early attempts to improve conductivity of NP solids, pyridine treatment has been used to replace the bulky ligand layer around NPs and shorten inter-particle spacing.64 Benefiting volatile property of pyridine, almost no gap between nanoparticles have been reported with gentle heat treatment, however, due to weak bonding strength, the ligand exchange yield is too poor and the repeated treatment is essential for this method. There are many other reports that shortening or removal of surface ligands of NPs decreases the interparticle gap. As such, hydrazine treatment reduces the gap from ∼1.7 to ∼0.3 nm.62 Other than interparticle spacing, mid-gap trap state inherited from surface dangling bonds 65 and low dielectric constants of NPs 66


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must also be considered as potential problems for the charge transport through the network of NPs (Section 4.2.1).

2.2. Nanowires, Nanotubes, and Nanofibers The processes for manufacturing solution-based nanowire (NW) and carbon nanotube (CNT) devices start with NW/CNT dispersions. They are often made using similar surfactants coatings as those used for NPs. The dispersions are then solution- or spin-casted onto the desired substrate. If the density of the NWs/CNTs is high enough to afford continuous conductivity over a network of conducting elements, a lithographic process is applied to pattern metallic contacts in the desirable geometry directly onto the dense film of NWs/CNTs. When the surface density of the conducting elements is low, one can make single NW/CNT devices.

In this case,

patterning can be performed before NW deposition or after. Alternatively, a conductive atomic force microscopy (AFM) tip, or metal microprobes are used as electrodes. 67 Although many NT/CNT devices have been demonstrated by this method,67,67 random orientation of NWs and CNTs on the substrate lead to poor device uniformity and low fabrication yields.

Thus,

alignment of NWs/CNTs on gate substrate emerged as one of the key factors of successful manufacture of NW circuits. The methods of direct growths of NWs on electrodes are reported as well, but for very limited materials and structures. 68,69,70 There are two main strategies for aligning or assembling NWs/CNTs for device manufacturing: to align pre-grown NWs or to assemble NWs at desired locations. Flow-assisted alignment,

71 , 72

bubble-blowing technique, 73 Langmuir-Blodgett (LB) layers, 74 , 75 , 76 , 77 , 78 , 79

contact/roll printing, 80,81 near field/gapping electro-spinning,82,83,84,85,86,87,88 and dielectrophoretic



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assembly 89,90,91,92,93 ,94,95,96,97 were used. In the flow-assisted alignment method, NWs/CNTs suspended in a solution are aligned by a shear force from the motion of a fluid. The NWs are aligned in the direction of the flow to minimize the fluid drag forces. For example, Huang et al.72 have aligned SiNWs by confining the fluid in a microfluidic channel (Figure 2A). This work shows that the fluidic-directed alignment readily extends over hundreds of micrometers. Even more complex NW structures of many different compositions such as GaP, InP, Si, CdS, Ge, and single-walled CNTs have been made by this technique. 98,99,100 However, the uniformity of shear force in a large channel is a challenging point of this technique. Yu et al.73 have suggested another method, the bubble-blown technique, in which the Si NWs, CdS NWs, CNTs are aligned along the shear force created by the expansion of the film. (Figure 2B) The uniqueness of this method is that films obtained by bubble-blowing can be transferred to both rigid and flexible substrates. The LB technique was applied to transfer monolayers of organic materials from solvent onto a substrate to make thin films utilizing the tendency of the solution to minimize their surface energy during compression. Whang et al.79 have successfully transferred onto substrate LB monolayers with a parallel array of Si NWs. The spacing in the parallel NW array also can be adjusted by the lifting speed and the pressure of the compression. Ag,101 Si,79 Ge,75 ZnSe,76 V2O5,102 VO2,78 PbS,103 as well as single-walled CNTs104 have been successfully assembled in large scale by this technique. In 2007, Javey et al. developed the contact-printing method, transferring vertically grown NWs from the growth substrate onto a lithographically patterned receiver substrate.80,105,81 Interestingly, the cylindrical growth substrates used for differential roll printing of NWs are also reported.81,106 The contact/roll printing method involves the directional sliding of the NW growth substrate. Aligned NWs, driven by directional shear force during the


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process, are eventually detached from the origin due to the van der Waals interactions of NWs with the receiver substrate (Figure 2C). Chang et al.106 reported a method using a PDMS stamp to improve density of arrays by re-transfer roller printing assisted ZnO-NWs on PDMS stamp to another receiver (Figure 2D). The process is generic for various NW materials, including Ge, Si, InAs, CdSe, and ZnO NWs. 107,80,105,108,81 Field-directed assembly, near field/gapping electro-spinning, and electrophoretic assembly represent promising methods for making nanoscale devices since they have the ability not only to align the NWs but also to assemble individual NWs at several independent locations simultaneously.89,95,90 Near-field or gap electrospinning82,85,86,109,87,88,110,111 can also be used to realize patterned deposition of nanofibers. Introducing an air gap into the conventional collector stretches jetting nanofibers across the gap to form a uniaxial parallel array by the force originated from the high-voltage splitting electric field (Figure 3A).112 In this gapping method, the spun fibers could be from polymers, 113 composite materials, 114 or semiconducting inorganic materials such as TiO2,83 SnO2,83 ZnO,112 Ag-ZnO.115 Since 1951, dielectrophoretic assembly (DEP) was one of the most effective bottom-up approaches to align or assemble particles. In DEP, the motion of polarizable suspended materials is controlled by the externally applied non-uniform electric ﬁeld. 116 This method typically utilizes positive dielectrophoretic force to align pre-grown NWs in parallel along the electrical field direction between electrodes or to assemble NPs into nano/micro bridges between electrodes. Various dielectric NWs such as Au,90 CdSe,93 CdTe,117 Si, Rh,71 Ag, Se,94 InP,118 ZnO,92, 119 CNTs, 120 , 121 as well as polymer nanofibers 122 have also been assembled by this approach. From the appropriate electrode design and adjusting parameters such as applied voltage, frequency, and dielectric constant of media, pre-grown NWs could be assembled at the



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desired location in the micro-device in high yield. For example, Freer et al.95 reported that single Si NWs could be assembled on 16,000 pre-patterned electrode sites by controlling the balance of surface, hydrodynamic, and DEP forces with 98.5% reliability (Figure 3B). Moreover, DEP assemblies of individual NPs can bridge micropatterned electrodes, eliminating a step of pregrowth or synthesis of NWs117 (Figure 3C). Simple devices in which DEP-assembled NP bridges, exploited as gas,123 optical,124 or biosensors,125 were demonstrated. Similarly to NPs, the problems with charge transfer in NWs/NTs networks are also associated with discontinuities of CNT-to-CNT and NW-to-NW charge conduction pathways . Packing density of NW/NT arrays and the electrical properties of the entire film are governed by alignment of NWs/CNTs.79 For efficient charge transfer though the NW/NTsfilm, it is important to reduce a number of hopping events through the film, rather than to regulate their packing density and inter-spacing. Greater length and coherent orientation of NWs/NTs minimize the number of hopping events required to collect charge carrier to electrodes. 126,127 2.3. Nanosheets Nanosheets are the nanoscale building blocks that are possibly most suitable for electrically conducting thin films while retaining the possibility of solution processing. Nanosheets have a comparable structure to atomic layers of other conventional deposition techniques such as molecular beam deposition. A rich spectrum of nanosheets have been made by chemical synthesis,128, 129 assembly from NPs,130,131 or delamination.132,133,134 Starting from a stable colloidal suspension of nanosheets, several strategies can be adapted to prepare hierarchically organized structures of nanosheet-based materials on various underlying electrode substrates.


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The simplest way to obtain the film with nanosheets is to evaporate a colloidal solution on the flat surface which involves some degree of their self-organization. Colloidal nanosheets tend to form films with uniformly flat organization because it corresponds to their energetic minima. 135,136 Therefore, even fairly simple techniques represented by drop-casting, dip-coating, or spraying results in highly organized multilayered structures. However, the films prepared by such procedures often tend to crack upon drying which disrupts the connectivity of the material at the micro- and macroscale.137 Spin-coating is one alternative method, which could produce continuous large-area films without nanometer scale wrinkling, much better overall quality, and homogenous thickness. 138 The thickness and homogeneity of the film have been known to depend on the colloidal concentration, spinning speed or acceleration, and the number of spincoating cycles. 139 Alternatively, vacuum filtration has also been used to deposit uniform layers of nanosheets for the preparation of film-based electrodes. This technique involves filtering the suspension that contains the nanosheets through a porous membrane with well distributed pores, such as a cellulose ester membrane, an anodic aluminum oxide (AAO) membrane, or an anodisc membrane filter. 140,141,142,143 Other techniques also take advantage of the ability of nanosheets to orient themselves with respect to the solid surfaces and produce films with organization favorable to X-Y plane charge transport. Layer-by-layer (LBL) assembly of nanosheets is one of the techniques utilizing this ability, and has been studied by several groups. 144 , 145 , 146 , 147 ,40 Sequential adsorption of nanosheets with oppositely charged macromolecules enables deposition of multilayers of nanosheets on solid surfaces or even at the liquid and air interface.148 LBL is convenient for the assembly of nanosheets that carry charge, while macromolecules are often represented by oppositely charged polyelectrolytes. This is a typical realization of LBL assembly, but the



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presence of strong charge and the use of aqueous media, which could be problematic for electronic devices due to the corrosive properties of residual water, are not hard requirements. Along the film normal, film thickness could be controlled at nearly nanometer scale, which is better than conventional wet-processes such as the sol−gel method and is comparable to that of modern vapor-phase deposition techniques. Under optimized conditions LBL deposition yields nearly perfect mono- and multilayer films with nearly atomic accuracy, large area uniformity, and high density (Figure 4).147,149,150 Directed assemblies template by pre-existing patterns with alternating areas of conductors and insulators is also possible. Wei et al.145 reported the selective self-assembly of graphene oxide (GO) sheets onto an Au electrode pretreated with aminoterminated 11-amino-1-undecanethiol and subsequently lithographically patterned that leveraged the electrostatic attraction between GO and the amine terminal of the thiol and electrostatic repulsion with the similarly charged gold surface. Recently, several approaches for large-scale pattern of nanosheets onto various substrates, especially onto flexible ones, attracted extensive academic interest. Dua et al.151 employed inkjet printing technology to deposit reduced graphene oxide (rGO) platelets onto poly(ethylene terephthalate) (PET). Using aqueous surfactant-supported dispersions of rGO powder as the printing ink, inkjet-printed film has been successfully produced which also demonstrates acceptable electrical conductivities. In advance, the patterns were designed on a computer, which allows control of the film thickness by altering the number of passes and the gray scale on the input on the computer. “Micromolding in capillary”, reported by He et al.,152 is another example for fabricating GO-based patterns on various substrates including flexible ones. Using a polydimethylsiloxane (PDMS) stamp, the GO patterns made from capillary motion of suspension are formed on the surface-modified substrate after drying the whole system in a vacuum oven.


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Compared with other methods for the fabrication of such GO patterns, this method is fast, facile, and substrate independent. Organization of flexible 1-2 nm nanosheet films discussed above in the direction perpendicular to the substrate and along the film normal determines its conductivity in X-Y direction because the tunneling gaps are primarily localized between the sheets. These tunneling gaps can be significant enough to impede conductivity. As such, Sasaki et al. reported that titania nanosheet/polycation LBL films have interplanar spacing of 1.4−1.7 nm,147 while the thickness of the sheets was 1.2 nm; the gap between the sheets can be, therefore, estimated to be 0.2-0.4 nm, which enables charge transport but is still associated with considerable charge transport barrier (see Section 4). In general, LBL assembly exploits the electrostatic bonding between conductive nanosheets and oppositely-charged macromolecules, and can hinder electron transport resulting in rather low charge carrier mobility.153 Other types of assemblies including those from G and GO cannot demonstrate levels of conductivity comparable to those obtained with CNTs.154,155 The imperfections of the graphene crystal lattice and the same dilemma with insulating coatings that need to exist in solution but disappear in nanosheet assemblies also persist.

Practical realization of the solution-based nanosheet assembly methods was also

impeded by defects ranging from pinholes to overlapped patches, 156 responsible for both electrical and mechanical failures. 3. Typical Solution Processed Devices A large variety of microdevices have been realized by “bottom-up” construction with solution processed nanomaterials. Examples of these solution processed devices include fieldeffect transistors (FETs), diodes, sensors, light-emitting and photovoltaic devices, and other



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electronic devices. Here we shall review only some of these devices because the problems with electrical transport are very similar to all of them. The electrical properties inherent to all nanoscale materials such as size-dependent electronic structure, charging energy, tunneling gaps, and defect densities determine the quality of the material as a base for electronic devices. Thus, particular attention should be paid when considering different methods of assembly of nanoscale building blocks in thin films and their competitiveness as alternatives to vacuum-deposited semiconductor layers. As we shall see in all cases the major challenge is to transform the individual NPs separated by a gap into a film capable of transporting charge over microscale distances.

Sometimes this is accomplished by using electron/hole transport “connectors”

between the individual nanoscale building blocks, and sometimes it was achieved by realizing lattice-to-lattice connectivity between them. 3.1. Field Effect Transistors In the past decade, a large variety of semiconductor NPs have been used as the building blocks for thin film transistors.48,157,158,159,160,161 In general, the switching speed of a transistor is determined by the time required for the charge carriers to travel from the source to the drain electrode, thus charge transport through channel is critical for achieving high performance FETs. The first FET fabricated with NPs was been reported in 1999 by Jacobson et al. who used a postannealing procedure to sinter NPs as means to eliminate the interparticle gaps and acquire sufficient charge mobility in the film.162

Later, Talapin et al. successfully fabricated the first

NP-based transistor without heat treatment by post-synthesis replacement of bulky oleic acid surfactant with small hydrazine molecules.48 Regulating the length of surfactants could reduce inter-particle gap without the sintering process and provide enough charge transportability to the film. Since then, several other chemical treatments aimed at surfactant removal, 163 exchange,


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158,48 ,164,165,23,166,167

or crosslinking168 have been reported as methods for reducing gaps between

NPs, resulting in highly improved carrier mobility of NP-based films. With various treatments and design for surface ligands, a variety of NPs including PbTe,169,62 PbSe,48 ,164,170,171 PbS,157 SnTe,172 ZnO159,173,174,175 and other NPs have been used to fabricate film transistors that show reasonable conductivity at room temperature. Other than designing surface ligands to regulate gap between NPs, optimization of NP shape has been also intensively investigated for improving charge transport in the channel layer. Employing anisotropic nanomaterials such as nanorods, nanowires, or nanosheets has improved performance of nanomaterial-based FET devices by reducing the number of hopping events for charge carriers. For instance, Sun and Sirringhaus demonstrated that charge transport in spincoated ZnO films can be greatly improved as a result of increasing particle size and selfalignment of the nanorods along the substrate.173 They observed almost 2 orders of magnitude higher mobility in their FETs made of 65 nm-long, 10 nm-diameter ZnO nanorods when compared to the devices made from 6 nm spherical ones. By using octylamine as a stabilizer for ZnO, they achieved partial assembly of monodispersed ZnO nanorods in the transistor channel, and this alignment of nanorods resulted in improved FET performance. The performance of FET devices assembled of ZnO nanorods was further enhanced by the step of post-deposition hydrothermal growth when a thin layer of ZnO was chemically deposited into the voids/gaps between ZnO nanorods; the resulting devices achieved mobility values comparable with commercial devices. 176 Assemblies of nanoscale building blocks make possible engineering of electronic materials and metamaterials that was previously inaccessible for traditional synthetic methods. The materials made from different types of particles offers an intriguing opportunity for



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electronic devices because the make possible “mixing” the properties of semiconductor and metals at the level of electronic bands. In 2008, Lee et al. 177 investigated FET devices from core−shell particles with plasmonic Au core and semiconducting PbS shell. Two synergistic effects were observed in them; (a) extinction enhancement due to coupling of surface plasmon resonance in the Au core to the excitonic states in the semiconducting PbS shell, and (b) strong p-type electronic doping of Au-PbS nanocrystal solids that is explained by the intraparticle charge transfer between the PbS shell and the Au core. They also integrated superparamagnetic response in the transistor channel by combining semiconducting and magnetic functionalities of FePt and PbS or PbS with core−shell and dumbbell nanostructures. These NP-based FETs with multicomponent particles clearly show the benefit of the NPs-based electronics and the importance of developing a deeper understanding of electronic behavior both on intra- and interparticle levels. 3.2. Sensors Nanomaterials have been intensely studied for sensing systems due to their unique properties stemming from large surface areas, exceptionally strong effects on charge carrier transport, and comparable sizes with chemical and biological targets. 178 All these structural features enable improvements in sensitivity and selectivity over existing systems. Within the framework of this review we consider the sensors that utilize the changes in electronic transport through the materials assembled from nanoscale building blocks represented. For instance, transistor-based sensors that detect charged species by monitoring changes in the source-drain current (Isd) at fixed gate-source voltage (Vgs) for specific Isd when target molecules attach to the gate surface. In recent years, many types of semiconducting materials such as graphene, carbon


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nanotubes, and metal-oxide NWs, have been employed to develop the FET-based devices for chemicals, gas, and biomolecules as well as for pH sensing. Graphene attracted the interest of researchers as a foundation for sensors by taking advantage of thin-film transistor (TFT) architecture due to the potentially high carrier mobility and the low noise of the device attributed to the single continuous layer of graphene. Up to date, graphene FET (GFET) sensors were demonstrated to detect various gases including NOx,179, 151 NH3,179 H2, 180 , 181 , 182 H2O, 183 , 184 alcohol, 185 and H2S

186

.

Dispersion evaporation,152 spin-

coating,179 inkjet printing,151 and DEP assembly 187 described in section 1.1 were employed for manufacturing such devices. Dua et al. 151 showed the possibility of manufacturing a flexible sensor array by the inkjet printing. As for the other GFET sensors, the real-time detection of pH, 188, 189

heavy chemicals such as H+,190 and biomolecules including DNA,191,

192

glucose,193 and

proteins189, 194, 195, 196, 197 were reported. Note that in case of graphene, weak interlayer coupling is believed to lead to better device performance and the best devices were reported for singlesheet GFETs. 198,199,200

However, for devices based on variations of the conductance over a

network of graphene sheets, the dependence of performance on the degree of electronic coupling between the nanosheets, is likely to be opposite. In 2001, Lieber’s group in Harvard reported a highly sensitive FET based on NWs (NWFET) designed for detection of biological and chemical species: streptavidin, antibody, and the metabolic indicator Ca2+.201 Since then, the nanowire (NW)/nanotube (NT) FET sensors were studied intensively for real-time and label-free sensing capability. Both experimental and computational reports have demonstrated that the performance of the NW FET sensor is determined by scale and electrochemical properties of nanowire, such as diameters, carrier densities and mobilities, and surface chemistry.5 Other than semiconductor nanowires, metal



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oxide NWs such as In2O3-NW,

202

and ZnO-NW

203

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have been used to configure NW-FET

sensors. Since Kong et al. demonstrated the first CNT-based gas sensors in 2000,204 CNT-FET have been comprehensively studied for sensing not only gas molecules but also biomolecules, which involves various biomedical interactions such as antigen–antibody interactions, DNA hybridization, and enzymatic behavior. 205,206,207,208,209,210 Notably, a manufacturing process that can produce these sensors in large scale is still undergoing the process of development and the core issue that needs to be solved is related to reliable organization of NWs/NTs between the electrodes.

One might consider that the task of accurate positioning of single NW/CNT

between electrodes could be easier than that for NPs due to larger size, but the fundamental uncertainties of angular orientation of the NWs also results in much greater variability of electronic properties. For that reason the devices made from NW networks with efficient charge transport between the nanoscale conducting elements could actually be more advantageous than single NW FETs. 3.3. Photovoltaic Devices Conversion of solar energy to electricity by photovoltaic (PV) elements is the center of research programs in many groups. The core challenge of PV devices is to be competitive compared to the production of electricity from fossil fuels.211, 212, 213 In order to reduce the cost of PV devices, solution-based deposition of a large variety of solar absorber materials was explored over the past decade. Organic materials have been deposited by traditional solution methods and used to fabricate flexible PVs taking the advantage of low temperature processability. However, relatively poor electronic properties and photochemical stability of organic materials lead scientists to focus their search on solution processable hybrid and all-inorganic solar cells with nanocolloids.


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For hybrid solar cells, semiconductor NPs are used as either p- or n-type components, thus helping to extend the range of light absorption of the devices. Various NPs made from Si,214 CdSe,, 215 , 216 , 217 , 218 , 219 , 220 , 221 CdTe, 222 ZnO, 223 , 224 PbS, CuInSe2, 232 and HgTe

233

225 , 226 , 227 ,228 , 229

CdS,64,230 PbSe, 231

were used to prototype PV elements. In 1996, Greenham et al.64

studied the processes of charge separation and transport in composite materials formed by mixing CdSe or CdS NPs with the conjugated polymer poly(2-methoxy,5(2′-ethyl)-hexyloxyp-phenylenevinylene) (MEH-PPV) as the composite devices in which inorganic nanocrystals have a higher electron affinity than the conjugated polymers, are efficient in the separation of charges, and absorb a significant portion of solar radiation. To enhance electron collection, Huynh et al.221 employed rod-shaped CdSe which have a tendency to form directed chains, and defined a pathway to the appropriate electrode for the charge, which has been generated at the NP/polymer interface. Construction of solar cells only with inorganic NPs also has been demonstrated by matching the donor-acceptor pairs of different NPs. For instance, Gur et al. fabricated all inorganic PV cells with pyridine-treated CdTe and CdSe nanorods by spin-coating. In this system, CdSe and CdTe NPs can create the donor-acceptor pairs with a staggered band alignment, and photogenerated electrons at the interface can transfer to the CdSe phase. Cu2S NPs and CdS nanorod pairs have also demonstrated their solar conversion ability.234 However, these simple blending of inorganic NP configurations showed much poorer performance compared with hybrid ones, presumably because of inefficient particle-to-particle transport. Insufficient charge carrier separation can also contribute to the problem as well. The inefficient particle-to-particle transport and recombination loss of simple blending of inorganic NPs are generally originated from insulating barrier of surfactant and defect density



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around NPs. Minimization of interparticle spacing and defect density by employing new class of surfactants has been made one of the priorities for last decade. Utilizing relatively short organic ligands such as aromatic thiols,235 alkylamines,48 alkyltionols,236 and mercaptocarboxylic acids237, realization of inorganic NP photovoltaic has been reported with much enhanced efficiency. More recently, using monovalent halide anions as atomic ligand for NPs, Tang et al.288 have reported all inorganic solar cells with up to 6% of power-conversion efficiency. These halide anions such as Cl−, Br− and I− offer a shallower trap state distribution than the best organic ligands and size scales down to 0.1 nm whereas short organics such as ethanedithiol have ~0.5 nm. 3.4.Light Emitting Devices Narrow band and tunable range from UV to near-IR of emission from NPs allow them to play the role of emitters for thin film light-emitting diodes (LEDs).17, 18 19, 20 ,21, 22, 238,239,240,241 The NP-based LEDs, often named NLEDs or quantum-dot LEDs, QD-LEDs have remarkably improved over the past decade.238, 242, 243 In 1994, Colvin et al. reported the first LEDs utilizing CdSe NPs.17 They used TOPO-TOP capped CdSe NCs as the emitting and electron transporting layer and poly(p-phenylenevinylene) (PPV) as the hole transport layer (HTL). By tuning the NP size, the emission color of the LED could be easily changed from red to yellow. The luminescence efficiency, however, was fairly low (0.001−0.01%) due to imbalanced carrier injection into the NPs. Since this first NP-based LED, significant progress has been achieved in optimization of all their components,18, 19 but even after improvements, poor electron transport through the NP multilayers lead to imbalanced charge injection, and consequently, the relatively poor luminescence efficiency of the QD-LEDs compared to single-crystal and organic LEDs.


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To achieve higher charge injection, Bulovic, Bawendi, and co-workers introduced heavily doped electron transporting layers in order to form excitons in the polymer and nonradiatively transfer to, and recombine in, the semiconductor NPs.242 Their LED design in which a single monolayer of CdSe/ZnS core/shell NPs was sandwiched between hole- and electrontransport organic layers help to achieve high performance. In their devices, the organic layers transport charge carriers to the vicinity of the NP monolayer from which the luminescence originates. This is in contrast to previous studies that utilized NP multilayer films, on the order of 10–20 layers thick, which had the dual function of both transporting electrons and serving as the emissive layer. They observe a 25-fold improvement in luminescence efficiency (1.6 cd A-1 at 2,000 cd m-2) over the best previous NP-LEDs results. In 2007, Sun et al. reported highperformance LEDs with saturated red, orange, yellow, and green emissions, all utilizing CdSe/ZnS core−shell NPs.240 As compared to the Bulovic−Bawendi design, they used a layer of PEDOT:PSS between ITO electrode and HTL to improve the hole injection and used spin coated poly-TPD as the HTL. PEDOT:PSS was used as the buffer layer on the anode mainly to increase the anode work function from 4.7 eV (ITO) to 5.0 eV and to reduce the surface roughness of the anode to obtain stable and pin-hole-free electrical conduction across the device. Poly-TPD was used as the HTL in consideration of the fact that its highest occupied molecular orbital (HOMO) level is 5.2 eV, which is very close to the work function of the ITO/PEDOT:PSS anode, and also because it possesses an excellent hole-transport capability. Even more important, they found that the thickness of the NP layer determines the efficiency and luminance of LED and should be carefully optimized for each color, depending on the size and structure of NPs. They experimentally showed that thicker layers than 2 monolayer NP film exhibit low electroluminescence efficiency owing to the poorer charge transportation between NPs. Thinner



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emissive layers down to monolayer NPs, however, had increased leakage current through the QD layer without radiative recombination owing to the presence of voids, grain boundaries, and interstitial spaces in the NP monolayers. To become more competitive with other emerging and established technologies, such as organic LEDs (OLEDs), both brightness and especially lifetime of NP-based LEDs have to be considerably improved. 244 , 245 , 246 Higher stability of core−shell NPs and electron- and holetransport layers, optimization of the energy transfer, and carrier injection from organic molecules into the NPs are need to be considered to achieve these goals. Consideration must also be given to toxicity of Cd-based NPs typically used for LEDs. Recently, attempts to develop highly luminescent NPs from more environmentally friendly chemical compositions such as InP/ZnS core−shells247,

248

or CuInSe2 249, 250 have also been made. Toxicity of NP encapsulated in epoxy

resin in a device may appear to be of small consequence to electronics, however, it becomes significant when considering the cost of environmental compliance and biomedical devices including implantable electronics.251, 252 4. Understanding Carrier Transport in Solution Processed Devices Charge transport in materials composed of nanoscale building blocks discussed above has been extensively studied for several decades. For simplicity we shall focus this section primarily on NPs because the issues with charge transfer in solution-processed thin films originate primarily from the gaps between the nanoscale conducting elements, which are prevalent in NP solids and composites. Current methods of synthesis and assembly of NPs allow for the control of principle parameters governing the charge transport, such as energy levels, inter-NP couplings, and spatial distribution of NP in the solid and therefore they are convenient for the description of


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charge transport problems. In this section, we briefly overview several transport mechanisms using NP arrays as prototypical models, the principle charge transport processes, such as tunneling, hopping, and co-tunneling, and also address the recently evolving technique for regulating or improving charge transport in an array of various nanomaterials. 4.1. Tunneling Tunneling is an elastic and coherent process in which charges move from one lattice site to another, maintaining their initial energies. In the close-packed composite films and superlattices from NPs, charge transport in the weak electronic coupling of NPs proceeds by tunneling mechanism through interparticle medium, which behave as dielectric tunneling barrier. As the electronic structure model of individual metallic or semiconductor nanomaterials, the discrete quantum confined wave functions localized on individual NPs can be used. When the spatial arrangement of metallic or semiconducting NPs is in close proximity to each other, the wave functions localized on the individual sites can interact with each other, exhibit coupling behavior and be delocalized over several neighbors or throughout the entire array. Since individual NPs are separated by surfactant molecules in the array, their surface ligands (also known as stabilizers, or capping agents) play a dominant role in electrical transport. The coupling energy of a NP array can be expressed with β ≈ hΓ, where h is Planck’s constant and Γ is the tunneling rate between two orbitals of neighbors, and the tunneling rate of charge effective mass, ∗ , can be approximated as ∗ ∆/ħ / ∆

(1)

As one can see in this equation, reducing inter-particle distance, ∆, and barrier energy, ∆, by employing proper inter-particle medium or surface ligand on NPs, can increase coupling effect in



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the array, thus improving conductance of the NP solids/composites. A number of studies have demonstrated this relationship and emphasized the importance of tunneling through the weakly coupled nanoparticles in the films.253,254 For instance, Murray and co-workers 255 demonstrated strong dependence of the transport properties on interparticle separation and width of the tunneling barrier by employing alkanethiolates of different lengths as stabilizers. The film conductivities of their drop-casted gold NP film, σ, can be expressed as

(2)

At various temperatures, conductivities showed exponential dependencies on the number of carbons (n) of alkanedithiol linker molecules, where κn is the decay constant per n (Figure 5A) The length of the stabilizers determines the inter-NP gap, d, in their system, thus the observation efficiently shows the conductivity dominated by electron tunneling between NPs. This observation also can be expressed with the decay constant per unit length, κd, since d is proportional to n )

(3)

Typically observed values of κd and κn are consistent with those reported in single-molecule tunneling studies. 256, 257, 258 4.1.1. Coulombic Charging Energy In addition to the coupling energy, Coulombic charging energy, Ec, also plays an important role in determining electronic structure and transport properties of a NP array. Ec is the energy consumed to put an additional electron on the NP. 259, 260, 261 The charging energy of particles can be expressed as


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/ ! ! "

(4)

This energy generally increases with reducing particle radius, r, and the dielectric constant of medium, #$ , and particles, #% . When tunneling occurs through an array of weakly coupled NPs, tunneling of an electron from one NP to another creates the energy barrier, E, which is almost twice that of Ec in the process &' (&' &') ( &'*

(5)

known as “Coulomb Blockade” (CB) that was studied extensively using single NPs and their chains. 262 , 263 If the coupling energy is smaller than the thermal energy, there is only weak coupling between the NPs, and therefore carriers are able to thermally overcome the CB as the temperature increases. At absolute zero, otherwise, sufficient large external bias is needed to make current flow. The CB energy also increases as the diameter of the NP decreases. 264, 265 In the case of gold, the CB-hindered electron transport could be observed at room temperature for NPs smaller than 2 nm.265 Effect of charging energy on electron transport in solution processed electronic materials could be observed through the dependence of activation energies on NP size. Brust et al. observed faster cross-linking reaction of the smaller NPs when they prepared small pellets with 2.2 and 8 nm Au NPs. 266 They have experimentally determined the electrostatic activation energy of charge transport by measurement of specific conductivities at ambient temperature under a pressure of 2 bars, and found that 5 times larger activation energy of 8 nm Au NPs compared with 2.2 nm ones. Quinn et al.66 also reported the energies scaled with NP size when they prepared films by drop-casting NPs stabilized with 1-adamantanecarboxilic acid and hexadecylamine. Mild thermal annealing resulted in the reduction of interparticle separation



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from 2.8 to 2.3 + 0.2 nm. The dependence of conductivities on temperature in films made from 10.2, 6.8, and 3.8 nm CoPt3 NPs exhibited clear Arrhenius behavior typical for the NeugebauerWebb model,267 which describes a simple thermally activated charge hopping between adjacent particles. As expected from Eq. 4, the largest activation energy was observed in the film with the smallest NPs. 4.1.2. Cotunneling Averin and Nazarov first recognized a tunneling process whereby electronic charge is transferred through several neighbor particles cooperatively.268 This charge transport mechanism is called cotunneling, and long-range hopping of charge carriers in NPs network can bypass the energy barrier of CB via this process. It has two modes: elastic and inelastic. As shown in Figure 5B, when an electron tunnels into a NP, another electron may concurrently escape from the same particle to another site.260 Elastic cotunneling is the dominant mechanism for the hopping conductivity at low temperatures T / Tcross, while at higher temperatures T 0 Tcross, electron transport occurs via inelastic cotunneling processes. The characteristic temperature Tcross of the crossover from the elastic to inelastic tunneling is given by Tcross 1 2

(6)

where Ec is the charging energy (E4 5 6 /48#$ #% 9 and δ is the energy level spacing of the NP (δ = (vV)-1, where V is the volume of the NP and v is the density of states at Fermi energy). Since Tcross for nanoparticles array is almost below 1K,269 the elastic cotunneling becomes significant only at very low temperatures.


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The tunneling probability should fall off exponentially with the distance,270, 271, 272 which is equivalent to the exponentially decaying probability of tunneling between states near the Fermi surface in the theory of Mott, Efros, and Shklovskii.273, 274 Thus, the hopping conductivity in granular materials including those made from NPs and other nanoscale building blocks can be expressed as : ; ?@ /?@.A B

(7)

where T0 is a characteristic temperature depending on the particular microscopic characteristics, and given by C@

DE " FG H

D;

(8)

II@ FG H

where C 2.8.274 and ξ is a localization length. The localization length is the decay length of localized average eigenstates ψ(x); because the probability distribution of ψ(x) has long tails, other averages of ψ(x) would give different values of the decay length, e.g. / J~5L ||/ 2N).275 For weakly coupled nanoparticles array, N O the NP diameter 29, and expressed by H

Z FG ?

(9)

"

where s is the hopping distance.272 4.2.

Charge Transport in Nanomaterials Array

4.2.1. Nanoparticle Solids Despite substantial progress in NP synthesis, fabrication of solid-state devices from nanocolloids and NPs in particular remains challenging. Roll-to-roll processes, spin coating, dip coating, inkjet printing, and similar solution-based techniques create substantial advantages in



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device processing but the performance of these devices still need to favorably compare with devices made by conventional inorganic crystalline. Solution-processed materials generally contain significant degrees of disorder of crystalline domains; electron transport within these domains, as well as at their boundaries, requires attention and optimization. Sintering individual NP arrays into crystalline phase results in improved film conductance and an impurity-free state, but the structural defects that limit the device switching speeds and incompatibility for flexible plastic substrates is problematic to use this high temperature process. Thus, research in solution processed electronics at low temperature emerged as the predominant future pathway to costeffective and flexible devices. Major impediments of electronic conductivity in NP arrays have been identified as poor exchange coupling and charge-carrier trapping due to highly insulating barriers around each NP. Charge transport in an array of NPs, separated by insulating capping ligands, depends on interparticle distance (the length of capping ligands), matching of the energy levels of neighboring NPs (site energy), the exchange coupling energy between the NPs, and on the Coulomb charging energy of the NP array. For efficient charge transport, the dispersion of site energies should not exceed coupling energy. In addition, if the coupling energy is smaller than charging energy, the NP array can behave as a Mott insulator. As described in Section 2.1, an individual NP can be described as a site-localized wave function. Therefore, for an array of strongly coupled NPs, the discrete wave functions on an individual NP can form a band. When the interparticle spacing decreases, the bandwidth increases as shown in Figure 5C. If the coupling energy exceeds the charging energy, the Coulomb gap disappears, and carriers can move freely throughout the NP solid, which is known as the Mott metal–insulator transition (MIT). Above the transition point, the coherent molecular-type orbitals extend over many NPs in


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analogy to an ordinary crystal. The onset of electronic delocalization is observed by dramatic changes in the optical and electronic properties of the metal NP solids. One possible method of coupling in NP networks is dipolar interaction between either permanent dipoles on highly polarizable NPs or transition dipoles resulted from oscillations in charge distributions as NPs change eigenstate.55 The highly polarizable NP induces permanent dipoles as they polarize neighbors, and this spontaneous reaction leads to formation of ferroelectric or antiferroelectric phase transitions in NPs solids depending on their configuration.276 Coulomb coupling among permanent dipolar NPs is known to be manifested in the spectrum of collective mode and appear some unique properties such as optical absorption at far-infrared frequencies.277,278 Interaction between transition dipoles can lead to electronic energy transfer at inter-particle separations between 5 and 100 Å, and tunneling between neighboring NP gives rise to dark and photoconductivity. 279 As decrease in the inter-particle distance, those solids reflect pure electronic coupling. After the first publication by Vossmeyer et al. 280 for closely packed layer of CdS nanoparticles, theoretical 281 and spectroscopical studies 282 , of dipole-dipole interactions have been done and applied in system of various NP solids.61 To enhance electronic coupling between NPs, replacement of bulky and insulating organic ligands with smaller molecules is possibly preferred to their complete removal of surface ligands because the latter generates multiple surface dangling bonds and midgap charge-trapping states. FETs made by Talapin and co-workers have the conductive channels assembled of closepacked semiconductor NPs. Replacement of oleic acid capping PbSe NPs with small hydrazine molecules increased the conductivity of PbSe NP films by ∼10 orders of magnitude. Mild chemical treatments with dilute hydrazine solutions, phenylenediamine, and other crosslinking molecules can be used to reduce the interparticle separation, strengthen electronic coupling



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between NPs, or attach them to electrodes or surfaces. Guyot-Sionnest and co-workers demonstrated a considerable improvement of charge transport in CdSe NP solids by crosslinking individual NPs with various aliphatic and aromatic diamines.283 The cross-linking of the NPs, followed by electrochemical charging with several additional electrons per NP, and the charge screening by the electrolyte's mobile ions, substantially reduces Coulomb energy. The mobility increases as a consequence of trap filling and the participation of multiple quantum confined electronic states (1S, 1P, etc.) in charge transport.48 Lastly, the use of specially designed degradable surfactants is another promising strategy. As such, tetrazoles can thermally decompose with formation of only gaseous products at relatively low temperatures compared with the sintering process.163 Although introducing shorter or linking molecules lead to great advances in electron mobility, these approaches also held the possibility of oxidation of the NPs, resulting in instabilities of the electronic properties. Using conductive conjugated polymers and oligomers such as end-functional polythiophenes or derivatives of poly(para-n-phenylene vinylene) were applied to improve charge transport and film morphologies for PV cells and LEDs.284 Based on similar mechanism, small and electronically transparent inorganic ligands such as metal chalcogenide complexes (MCCs) 285 , 286 and other charged small ions 287 , 288 have also been reported. In 2009, Kovalenko et al.285 have developed novel approach to create all inorganic NP solids by serving MCCs as convenient ligands for NPs. MCCs ligand such as Sn2S64−, In2Se42-, In2Se42- have relatively short ligand length, approximately 7 nm,288 and have shown greatly improved charge transport in NP solids since the ligand can act as “electronic glue” 286 in the network. However, processing MCC capped NP solids still requires thermal annealing to cure dangling bonds to remove deep traps in the film. In terms of performance, conventional


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semiconductor technology still preferred pure, all inorganic, highly crystalline materials, which indicated that a new class of solution processed high crystalline devices, should be considered rather than the one based on an individual NP array. 4.2.2. Charge Transport in Nanowire, Nanotube, anf Nanofiber Arrays The one-dimensional nature of NWs or CNTs provides better directionality to charge transport in solution-processed nanoscale materials, resulting in improved conductivity compared with the ones prepared from spherical NPs. One can also consider the increase of the aspect ratio of conducting elements to be the generic method for increasing efficiency of electron or exciton transport in solution-processed solids. In 2002, Huynh el al. reported high power conversion efficiency in dye-sensitized solar cells by employing CdSe nanorods.217 CdSe nanorods allow the system to achieve higher light conversion efficiencies than ones with spherical CdSe NPs due to the smaller number of interparticle hopping events required to collect electrons to electrodes. Wang et al. also employ CdSe NWs to fabricate high-performance hybrid photodetectors on both rigid and flexible substrates. Based on P3HT:CdSe NW heterojunctions, they have shown an enhanced photoresponse and stability utilizing the high hole-transport rate of P3HT and the high electrical conductivity of CdSe NWs. 289 By employing drop-casting of a solution of P3HT and CdSe nanowires, they also constructed flexible hybrid photodetectors on PET and printing paper. This efficient transport property of nanowire structure has been also demonstrated in optoelectronic devices with other semiconductor NWs such as ZnO NWs 290,291, 292 andSnO2 NWs.293 Gubbala et al.293 show SnO2 NW-based dye sensitized solar cells exhibit an open circuit voltage of 560 mV, which is 200 mV higher than that using spherical SnO2 nanoparticle based cells.



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CNTs are widely employed in electronic devices due to their high electrical mobilities and ballistic transport characteristics. To achieve coherent charge transport through films, wellaligned arrays of CNTs are a tremendously important issue to successfully fabricate the devices. Aforementioned assembly techniques in Section 1.1.2, including mechanical shear forces100,73,294 and electric295,296,297,298,299,300 field are applied to align CNTs, thus avoiding tube–tube contacts, which can limit charge transport through films. For the films including many CNTs, the electrical transport characteristics of the film can be controlled by not only their degree of alignment but also the length and diameter of the CNTs. For conductive films, long and relatively large diameter CNTs, which can minimize CNT–CNT junctions in transport and the band gap of single CNT respectively, are preferred.126,127 These studies show that although utilizing CNT films as conductive materials looks simple, the overall properties of the film has complex parameters related to their electrical characteristics including average tube length, tube diameter, and deposition method. 301,302 For polycrystalline semiconductor materials, band bending can occur mainly at the grain boundaries due to interfacial traps.303,304,305,306 Single crystalline semiconductor materials without grain boundaries, however, also have band bending at their surfaces or interfaces such as semiconductor/dielectrics, 307,308 semiconductor/electrodes, 309 semiconductor/electrolytes, 310 and semiconductor/chemisorbates. 311 In the same vein, the surface states or defects on single crystalline NWs can induce carrier trap energy levels, thus becoming the limiting factor in device performance.312 Hong et al. and Critchley et al. showed that the electronic transport in a NW can be influenced not only by their size and shape but also their surface morphology associated with the interface roughness in the system.313,314 The FETs made from smooth ZnO NWs with a larger diameter exhibited negative threshold voltages, indicating n-channel depletion-mode behavior,


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whereas those made from corrugated ZnO NWs with a smaller diameter had positive threshold voltages, indicating n-channel enhancement-mode behavior. This report indicates that the properties of nanostructures used as building blocks for the assembly of nanoscale devices strongly depend on their size, shape, and also surface morphologies. Therefore, the synthesis of surface morphology-and size-controlled nanostructures facilitates the fabrication and fundamental study of nanoscale devices with unique electrical properties. 4.2.3. Charge Transport in Nanosheets Stacks As described in Section 1.1.3, sheet-like nanomaterials including inorganic, graphene, and graphene oxide nanosheets are some of the most suitable building blocks for designing wellcontrolled films for microdevices. Similar to the one-dimensional nanostructures in the above section, confined structure of nanosheets can provide the directionality of charge transport path as well. Zu et al. have presented highly efficient dye-sensitized solar cells based on hierarchical ZnO nanowire−nanosheet architectures. 315 They have prepared ZnO nanosheet arrays and chemically grown dense ZnO single-crystalline nanowires on the surfaces of the primary ZnO nanosheets to enlarge internal surface area within the photoanode. With this architecture, a better dye loading and light harvesting on the nanowires follows by charge transport thorough the nanosheets. Figure 6 shows the possible electron transport path in this photoanode consisting of the hierarchical nanowire−nanosheet architectures. Due to an unmatched combination of sheet resistance and transparency of graphene, this material has attracted the interest of scientists for use as the building block for transparent electrodes in thin-film optoelectronic devices such as solar cells and light-emitting diodes. In principle, since graphene sheet is a zero-gap semiconductor with a very high Fermi velocity,



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183,316 , 317 , 318 , 319 , 320

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they have very high in-plane conductivities and charge carriers in them

delocalize over the entire sheet and can travel thousands of interatomic distances without scattering. However, since both transmittance and sheet resistance decrease with increasing film thickness, the study of graphene film thickness optimized for both the sheet resistance and the photon out-coupling efficiency. Similarly to devices utilizing NP or NW arrays, nanosheets can also be assembled to produce hybrid devices exploiting synergetic effects from different nanosheet materials. In 2009, Manga et al. reported the ultrafast electron transfer and photoconversion properties in the multilayer hybrid films composed with graphene and titania nanosheets.321 They fabricated selfassembled multilayer thin films consisting of alternating titania and GO nanosheets by LBL method. Interestingly, exposure of the multistacked films to UV light allows the photocatalytic reduction of the GO into graphene. Due to the functional separation of layers at the nanoscale, layered materials can be phase-segregated spatially to maintain unique 2D characteristics. 5. New Opportunities to Improve Charge Transport in Solution Processed Materials As described in the previous sections, bottom–up solution processed devices as a manufacturing concept still requires the need to overcome substantial technical and fundamental roadblocks.

Although

advances

involving

integration

of

various

solution-processed

nanomaterials in devices, their deficiencies compared to vacuum-processed, top-down patterned devices are still sizeable. In this section, we shall try to identify new approaches that could significantly enhance value for solution processed devices going forward. Among many potential structures for PV cells, the systems where semiconductor NWs are oriented vertically to the substrate appear to be the most suitable. However, many methods of


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solution processing are limited to the film formation and alignment of conducting elements along the substrate surface. The existing examples of nanoscale devices with “vertically” oriented features 322,323,324, 325 as well as the theoretical considerations presented above indicate that selfassembly of such structures could be highly beneficial to their performance. We envision a number of ways to improve device performance as additional methods for assembling nanostructures perpendicular to the substrate surface are discovered. This is especially true for those that afford fast charge transport, preferably via overlapping wavefunctions of NPs (Section 4.2.3). In more general terms, imparting solution processing with the ability to produce 3D featured films is likely to play the key role for realizing advanced colloidal processed devices.

As described above, the high barrier related to Coulombic charging energy (Section 4.1.2) significantly impedes charge carrier transport in assembled nanomaterials. Reduction of Coulombic charging energy can be obtained via improving lattice-to-lattice connectivity between NPs. For successful competition with vacuum-processed devices, NP solids should approach epitaxial connectivity through the creation of a nanoscale monocrystalline network. Indeed, Baumgardner et al. have reported increased PbSe NP film conductivity by several orders of magnitude after epitaxial fusion between adjacent particles. Lattice-to-lattice contacts without epitaxial match would be the next best option although such charge transport pathways will increase scattering of charge carriers.314

It may be considered a tall order for the realization of both epitaxial and non-epitaxial lattice-to-lattice connectivity between the NPs in a solution-based process without elevating the



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temperature as the semiconductor material starts annealing (ca. 350 ℃ ).162 However, the spontaneous crystallization NPs into NWs at room temperature is possible, as it was observed for CdTe NPs after partial removal of the stabilizer. 326 Analogical recrystallization behaviors, or lattice-to-lattice connectivity, of NPs are observed for FeS2, 327 PbS, 328 PbSe, 329 and more recently for Cu2S.330 This potentially unexpected transformation from individually dispersed NPs to epitaxially assembled nanoscale conducting networks can be realized with decreased temperature of phase transitions and recrystallization typical for all nanomaterials. Additionally, an ion exchange between NPs takes place when they approach each other beyond a certain gap distance.

The conventional wisdom for solution-processed devices is that the charge transport in thin films assembled from NW/CNTs and nanosheets is typically higher in the devices assembled from high aspect ratio colloids (aspect ratio >10) than those from small spherical NPs (aspect ratio ~1).

Along with the experimental data proving this general concept, and

demonstrating more efficient transport of electrons, or excitons, from NWs than for NPs,315 recent studies indicate that this can be challenged, especially for solution-processed devices that will need to experience the potentially large deformations exemplified by flexible electronics. It was found that stretchable composites based on Au NPs show unusually high conductivity, especially at high strains. Importantly the conductivity, charge carrier mobility, and charge concentration in the case of NP composites was higher than for high aspect ratio nanocomponents (Figure 7A). This effect is associated with the ability of NPs to self-assemble into conducting pathways in solid state. These experimental results show that as strain increases, NPs demonstrate higher conductance values than calculated by classical percolation theory. The


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NPs gradually re-organized into bands along the stretching direction, forming cellular-like selfassembled NP patterns observed under high stress (Figure 7B). Conductance values for high strain requires a new version of percolation theory that takes into account the ability of NPs to self-assemble, in order to be accurately described mathematically.

The ability of solution processes to easily mix and match nanomaterials with different compositions and dimensions can now be employed more often, considering the improvements in charge transport. As such, graphene nanosheets have an excellent electrical conduction in two dimensions (Section 4.2.3). A recent study shows the enhanced power conversion efficiency of dye-sensitized solar cells by reducing the charge recombination occurred at the various interfaces of the device, such as fluorinated-tin oxide NPs, and titinium oxide (FTO/TiO2), TiO2/TiO2, TiO2/electrolyte by spincoating graphene.331 As shown in Figure 8A, the graphene provides a more efficient current path to the electrode than the one without graphene coating. The remarkable power conversion efficiency was increased from 5.80% to 8.13%. Figure 8B shows that the graphene at TiO/FTO can be an electron acceptor and increase the charge transfer speed.332 Such incorporation of 2D nanomaterial with NPs can be extended to other electronic devices, especially when using graphene as an electron acceptor and charge transfer medium.



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Kim and Kotov. Figure 1

Figure 1. (A) TEM images of self-assembled NP superlattices: (a) 7.5 nm PbS nanocrystals; inset shows optical micrographs of “supercrystals” grown from PbS NPs. (b) Smectic ordering of 29 nm long, 4.5 nm diameter CdS nanorods. Reprinted with permission from Ref 255. Copyright 2010 American Chemical Society (B) TEM images of the characteristic projections of the binary superlattices, self-assembled from different NPs, and modelled unit cells of the corresponding three-dimensional structures. Reprinted with permission from Ref 60. Copyright 2006 Nature Publishing Group


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Figure 2. (A) Schematic of fluidic channel structures for flow assembly. Reprinted with permission from Ref 70. Copyright 2001 the American Association for the Advancement of Science. (B) Illustration of (i) a nanowire polymer suspension, (ii) bubble expansion over a circular die and (iii) films transferred to crystalline wafers, plastics, curved surfaces and open frames. Reprinted with permission from Ref 71. Copyright 2007 Nature Publishing Group (C) Overview of 3D NW circuit integration by contact-printing method. Reprinted with permission form Ref 103. Copyright 2007 American Chemical Society (D) Schematic illustration of the rolltransfer printing process. Reprinted with permission from Ref 104. Copyright 2009 IOP Publishing



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Figure 3. (A) Schematic illustration of the gapping method setup for electrospinning. Reprinted with permission from Ref 110. Copyright 2009 American Chemical Society (B) Optical darkfield and DUV images of nanowires assembled onto electrodes on a 4-inch quartz substrate after the completed process. Reprinted with permission from Ref 93. Copyright 2010 Nature Publishing Group (C) Schematic diagram of experimental setup for DEP and FESEM images after DEP of CdTe NPs solution. Reprinted with permission from Ref 115. Copyright 2011 the American Chemical Society


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Figure 4. Cross-sectional high-resolution transmission electron microscopy image of multilayer nanosheets (n = 5) on a SRO electrode. Reprinted with permission from Ref 148. Copyright 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim



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Figure 5. (A) Plot of 70 (○), 30 (●), and −60 °C (▼) conductivity vs number of carbons (n) in the alkanethiolate chains of Au309(Cn)92 monolayer-protected clusters. Reprinted with permission from Ref 251. Copyright 2000 the American Chemical Society (B) Schematic energy diagram for cotunneling processes: (a) elastic cotunneling, entering and exiting electrons have the same energy; (b) inelastic cotunneling, the entering and exiting electrons have different energies. During the inelastic cotunneling, an electron−hole excitation is generated in the grain. Reprinted with permission from Ref 320. Copyright 2007 American Physical Society (C) Evolution of energy levels in a lattice of hydrogen atoms during Mott metal–insulator transition (MIT) Reprinted with permission from Ref 256. Copyright 2008 American Physical Society


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Figure 6. Schematic diagram of the possible electron transport mechanism in the Dye Sensitized Solar Cells (DSSC) photoanode, consisting of the hierarchical ZnO NW−nanosheet architectures. Redrawn with permission from Ref 302. Copyright 2010 American Chemical Society.



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Figure 7. (A) Conductivity as a function of uniaxial strain of LBL (red) and VAF (blue) composites. (B) Temperature dependence of conductivity for LBL and VAF. (C) SEM image of focused-ion-beam milled VAF assembled films at 200% strain. Reprinted with permission from Ref 323. Copyright 2010 Nature Publishing Group.


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Figure 8 (A) Schematic illustration of the devices and electron transport path. Left: depicting the reference DSSC with only TiO2 nanoparticles as anode semiconducting materials and N719 dye as sensitizer; Right: illustrating DSSC with graphene as an interlayer of FTO/TiO2 and in TiO2 nanoparticle network. Reprinted with permission from Ref 318. Copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, (B) Operational principle of the device: the introduced 2D graphene bridges perform as an electron acceptor and transfers the electrons quickly. Hence, the recombination and back reaction are suppressed. Reprinted with permission from Ref 319. Copyright 2010 American Chemical Society.



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