Competition between Charge Transport and Energy Barrier in

Oct 27, 2014 - and Byoungnam Park*. ,†. †. Department of Materials Science and Engineering, Hongik University 72-1, Sangsu-dong, Mapo-gu, Seoul ...
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Competition between Charge Transport and Energy Barrier in Injection-Limited Metal/Quantum Dot Nanocrystal Contacts Youngjun Kim,†,○ Seongeun Cho,†,○ Sunho Jeong,‡ Doo-Hyun Ko,§ Hyungduk Ko,§ Namho You,∥ Mincheol Chang,⊥ Elsa Reichmanis,⊥ Jun-Young Park,# Sung Young Park,‡ Jong Suk Lee,*,∇ Heesun Yang,*,† Insik In,*,‡ and Byoungnam Park*,† †

Department of Materials Science and Engineering, Hongik University 72-1, Sangsu-dong, Mapo-gu, Seoul 121-791, Korea Department of Polymer Science and Engineering, Korea National University of Transportation, Chungbuk, Chungju 380-702, Korea § Center for Optoelectronic Convergence Systems, Korea Institute of Science and Technology (KIST), Seoul 136-791, Korea ∥ Carbon Convergence Materials Research Center, Institute of Advanced Composites Materials, Korea Institute of Science and Technology, Jeollabuk-do 565-905, Korea ⊥ School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, NW, Atlanta, Georgia 30332-0100, United States # HMC & Green Energy Research Institute, Department of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 143-747, Korea ∇ Center for Environment, Health and Welfare Research, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136-701, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Injection-limited contacts in many of electronic devices such as light-emitting diodes (LEDs) and field effect transistors (FETs) are not easily avoided. We demonstrate that charge injection in the injection-limited contact is determined by charge transport properties as well as the charge injection energy barrier due to vacuum energy level alignment. Interestingly, injection-limited contact properties were observed at 5 nm diameter lead sulfide (PbS) quantum dot (QD)/Au contacts for which carrier injection is predicted to be energetically favorable. To probe the effect of charge transport properties on carrier injection, the electrical channel resistance of PbS nanocrystal (NC) FETs was varied through thermal annealing, photoillumination, ligand exchange, surface treatment of the gate dielectric, and use of different sized PbS NCs. Injection current through the PbS/Au contact varied with the FET mobility of PbS NC films consistent with a theoretical prediction where the net injection current is dominated by carrier mobility. This result suggests that the charge transport properties, that is, mobility, of QD NC films should be considered as a means to enhance carrier injection along with the vacuum level energy alignment at the interface between QD NCs and metal electrodes. Photocurrent microscopic images of the PbS/Au contact demonstrate the presence of a built-in potential in a two-dimensionally continuous PbS film near the metal electrodes.

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INTRODUCTION

(FETs), and photodetectors, QDs interface with inorganic

Nanocrystal (NC) quantum dots (QDs) have attracted much interest due to their unique electrical and optical properties.1−4 Size-dependent NC QD energy band gaps offer flexibility in optimizing energy levels, thereby controlling charge transfer between QDs and other organic or inorganic materials when assembling optoelectronic devices such as QD photovoltaics.5−8 In light-emitting diodes (LEDs), field effect transistors © 2014 American Chemical Society

electrodes including metal and transparent oxide materials through which charge carriers are injected.9,10 Received: July 26, 2014 Revised: October 9, 2014 Published: October 27, 2014 6393

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Figure 1. Structural and electronic properties of PbS NCs. (a) A schematic diagram of a PbS FET and TEM image of PbS NCs. (b) UV−vis absorption spectrum of PbS NCs. (c) Plot of absorption coefficient (α) as a function of excitation energy (hν). The energy band gap of PbS NCs dispersed in chloroform was estimated from a linear fitting of the plot. The inset shows an energy band diagram for a Au/PbS (5.5 nm)/Au structure. (d) Cyclic voltammetry results for PbS film deposited onto a Pt electrode. The scan rate was 0.1 V/s. (e) AFM tapping mode height images for a PbS film deposited using blade coating.

alignment between the component layers.11,12 It has also been reported that localized states at the interface can modify the carrier injection barrier.13,14 In other words, theoretical predictions including Schottky−Mott theory based on the

In the devices, charge injection through the metal electrodes is unavoidable for operation. Through extensive research, charge injection efficiency is found to be governed by the energy injection barrier determined by vacuum energy level 6394

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Figure 2. ID−VG transfer characteristic curves in the linear transport regime of transistor operation for EDT and BDT treated PbS films with a schematic of a bottom-gate FET structure. The channel length and width of the FET devices were 50 μm and 1 mm, respectively. The drain current was fixed at −5 V. The molecular structures of EDT and BDT are displayed.

in combination with UV−vis absorption spectroscopy. The absorbance spectrum of PbS NCs in Figure 1b shows that the first excitonic absorption energy is 1.08 eV. In Figure 1c, the band gap (Eg) of PbS NCs dispersed in chloroform solvent was calculated from the plot of absorption coefficient, α2, as a function of excitation energy of light, hν, according to the equation, α2(hν) = C(hν − Eg), in which C is a constant, producing an energy band gap of 0.84 eV.20 Optical transitions in insulator show that the absorption coefficient is a parabolic function of the excitation energy of light.21 In theory, C depends on the refractive index of the material. The HOMO level of PbS was calculated using CV as shown in Figure 1d. From the oxidation onset potential at 0.43 V with the energy band gap from UV−vis absorption spectroscopy, the HOMO and LUMO levels were calculated to be −4.80 and −3.96 eV, respectively. From the energy levels for Au and PbS NCs given in the inset in Figure 1c, hole injection is favorable while a hole extraction barrier of 0.3 eV is predicted. From the calculated energy band gap, the size of PbS NCs, d, was approximated to be 5.5 nm through the empirical equation, Eg = 0.41 + (1/ (0.0252d2 + 0.283d)).22 The approximate PbS NC size of 5.5 nm from the equation was consistent with the average values, 5.4 ± 0.1 nm, obtained from the several TEM images. For our experiments, three different sizes of PbS NCs, 5.04, 5.23, 5.51 nm, were used. Figure 1e shows the PbS NC film morphology, prepared with a blade-coating method, without ligand exchange. In the AFM images, PbS NCs formed islands in which the step size of a single layer was ∼5.9 nm, close to the size of PbS NCs (5.51 nm) obtained from TEM and UV−vis absorption spectroscopy. Carrier transport properties in a PbS NC film can be probed using a bottom-contact FET device shown in the inset of Figure 2a. The majority carriers in PbS NC films treated with ethanedithiol (EDT) or 4,4′-biphenyldithiol (BDT) are holes as shown in the FET transfer characteristic curves (ID−VG) in Figure 2. Drain current, ID, is determined by the mobility, μ, carrier concentration, Cox(VG−VT), in an FET device with the width of Z and the channel length of L as given by eq 1:23

energy level difference are often limited by surface states, that is, defects due to structural disordering at the interface.15,16 These findings imply that understanding the electrical contact properties is no longer simple. Indeed, for disordered organic semiconductors in contact with metal electrodes, charge carrier injection is affected by charge transport properties of the organic materials because back diffusion of carriers from the organic semiconductors to the metal electrodes decreases the net current.17 For QDs, their surface states arising from a high surface to volume ratio complicate analysis of charge injection at QD/ metal contacts.18,19 Thus, theoretical predictions based on vacuum energy alignment must include structural defects and low carrier mobility, necessitating study of the correlation between charge transport and electrical contact properties. Simultaneous study of electrical contact properties at NC/ metal electrodes and charge transport properties in NC films can provide insights into understanding the charge injection process, which is crucial in operating QD electronic devices. In this report, a lead sulfide (PbS)/Au contact as an injection-limited contact is demonstrated through a diode arrangement in which a PbS film is sandwiched between Au and ITO electrodes. We show that charge transport properties in PbS films are dominant over the carrier injection barrier for determining the magnitude of the injection current. The channel resistance, inversely proportional to carrier mobility, is correlated with the electrical contact properties, that is, electrical contact resistance extracted from multiple voltage probes between the source and drain electrodes. To explore the electronic and structural properties of the contact and channel, cyclic voltammetry (CV), photocurrent microscopy (PCM), atomic force microscopy (AFM), and scanning electron microscopy (SEM) were used.

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RESULTS AND DISCUSSION

PbS NC size and their respective lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels were characterized by transmission electron microscopy (TEM) imaging (Figure 1a) and CV measurements 6395

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Figure 3. Injection-limited ITO/Au/PbS/Al diode structure. Plot of the current density as a function of voltage applied to the ITO side is shown with a schematic diagram of the diode.

ID =

Z μCox(VG − VT)VD L

Despite the favorable energy level alignment for hole injection as shown in Figure 1c, PbS/Au contacts are found to be injection-limited through comparison between the measured current and the space charge limited current estimated from theoretical prediction. The diode arrangement, in which holes are easily injected and extracted through Au and Al, was used to probe hole injection properties, eliminating the reverse bias for extraction of holes in the two-contact Au/PbS/ Au device structure, in which a high energy barrier of 0.3 eV must be overcome for extraction of holes. In the forward bias, where a positive voltage is applied to the indium tin oxide side, hole carriers are injected into PbS, as supported by the FET measurements in Figure 2. Surprisingly, despite elimination of the reverse biased contact, the measured diode current (blue □) was much less than the space charge limited current (red ○) into which a sufficient number of carriers are injected (Figure 3), indicating that the PbS/Au contact is injectionlimited. This finding is also supported by comparison of the contact resistance and channel resistance obtained through two-contact devices with multiple voltage probes for which the magnitude of the contact resistance is comparable to or larger than that of the channel resistance. Current in an injection-limited device has been correlated to the carrier mobility in disordered semiconducting materials with low mobility in contact with metal electrodes.15 In the system, net current is governed by competition between the injection current from the metal electrodes to the semiconducting materials and the diffusion current from the semiconducting materials to the metal electrodes. According to Scott and Malliaras,17 the net current density in a device including injection-limited contacts, Jnet, is predicted to be determined by not only the injection energy barrier, ϕB, but also by the carrier mobility, μ, in semiconducting materials as represented by eq 2:

(1)

where Cox is the capacitance of the gate dielectric, VT is the threshold voltage, and VD is the drain voltage. If most of the charge carriers are mobile (free) in the electrical channel, the drain current density can be expressed by ID/A = neμE. In the linear region in which the lateral potential difference between the source and drain electrodes is assumed to be negligible due to a small drain voltage, electric field and the contact area are represented by VD/L and Zt, respectively. The two-dimensional carrier concentration independent of thickness t is given by Cox(VG−VT)/q. The FET mobility is calculated by the slope in the linear region of the ID−VG plot in which the drain current linearly increases with gate voltage. In other words, the mobility is a proportionality constant reflecting how fast mobile carriers can flow in the channel at a constant electric field. The threshold voltage is defined as the required voltage to induce mobile carriers in the channel. It can be extracted by extrapolating the ID−VG plot in the linear region to the gate voltage axis in which the number of mobile carriers are depleted, that is, ID = 0. As the gate voltage, VG, applied to the bottom gate electrode increases from 0 to −20 V, the drain current in the EDT- and BDT-treated FETs increases, indicating that the induced charge carriers in the channel are holes. The FET mobility and the threshold voltage were 3.1 × 10−4 cm2/(V s) and 3 V for EDT and 2.4 × 10−5 cm2/(V s) and 2 V for BDT-treated PbS FETs, respectively. Enhanced electronic coupling between PbS NCs due to a shorter ligand length of EDT in comparison to that of BDT accounts for a higher FET mobility in the EDT-treated PbS FET than that in the BDT-treated PbS FET.24 The positive threshold voltages for the FETs show that mobile hole carriers exist in the channel even without gating in the FETs. Space charge limited current, Jsch, results from a sufficient forward bias without being limited by charge injection. Therefore, for an injection-limited contact, the magnitude of space charge limited current would be larger than that of the measured current.15 Space charge limited current is represented by Jsch = (9/8)εε0μ(V2/L3) in which V is the potential applied, L is the vertical channel length, and ε is the dielectric constant of PbS.25,26 In a device possessing injection-limited contact, the number of injected carriers is much less than the number of thermally generated carriers in the electrical channel.15,27 Therefore, carrier injection limits the magnitude of the current through the sample.

Jnet = N exp( −ϕB /kT )eμF

(2)

where N is a constant, e is the elementary charge, k is the Boltzmann constant, T is the temperature, and F is the electric field applied. At equilibrium the net current Jnet = Jinj − Jrec is zero, indicating that the injection current is represented by the recombination current given by Jrec = n0eμE0. The carrier concentration at the metal/semiconductor interface, n0, is proportional to the number of carriers injected through the energy barrier ϕB. Therefore, the injection current is proportional to exp(−ϕB/kT) based on thermionic emission and mobility from the recombination process with the proportion6396

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resistance to replace the FET mobility. The channel resistance, Rch, was calculated from the relation, Rch = R0(L/Z), where R0 is the channel sheet resistance, R0 = (|V1 − V3|/I)(Z/d). V1 and V3 are the potentials measured near the source and drain electrodes, respectively, as shown in the inset of Figure 4a. The contact resistance was calculated by subtracting the channel resistance from the total resistance from I−V measurements at a constant drain voltage of 3 V. The contact resistance at the source and drain electrodes was determined by dividing the voltage drop at the electrodes by the drain current. The potentials measured along the channel for EDT and BDTtreated PbS devices were almost linear as shown in Figure 4a, suggesting that the PbS films in the devices are homogeneous over the channel area. The channel resistance was intentionally varied over a range of nearly 4 orders of magnitude using various methods as noted in the Experimental Section. The experimental conditions for all devices in Figure 4 are detailed in the Supporting Information. In Figure 4b, the channel resistance versus contact resistance plot was fitted to eq 2, producing Jnet ≈ μ−1.05, which is in good agreement with dependence of J on μ in eq 2. This clearly indicates that charge injection at the PbS/Au contact is limited by the charge transport properties in the PbS QD NC film rather than by the energy barrier. The electrical and physical contact properties at the PbS/Au contact were probed with PCM. Figure 5a shows an optical setup for PCM. The photocurrent measured across the channel and contact regions is displayed in Figure 5b in which enhanced photocurrent resulting from a built-in electric field is observed near the PbS film close to Au electrodes without biasing. The photocurrent over the entire region was mapped to image the local band structure near Au electrodes in Figure 5c. The two distinct colored regions at the source and drain contacts result from the photocurrent due to photoexcited electrons and holes guided by the built-in electric field. According to the sign of the photocurrent and the position of the HOMO as obtained from CV measurements,28,29 the energy band bending can be drawn as in Figure 5d. This is consistent with the energy band structure predicted by the CV method in which the Fermi energy level of PbS is positioned closer to the vacuum level. Additionally, the photocurrent mapping image near the Au electrodes in Figure 5c clearly shows that the PbS film is continuous near the source and drain electrodes. Thus, the injection-limited current in Au/PbS/Au devices clearly arises from a low hole mobility in PbS films and not from a discontinuous PbS film morphology, which could be the origin of a high contact resistance. Mobility-dependent current injection is predicted to occur to most of the QD films in contact with metal electrodes due to a high density of carrier trapping centers resulting in a low mobility. With the mean free path of the resistive QD film being far shorter than the channel length between metal electrodes, mobility becomes dominant over the injection barrier. As the mobility increases, the Richardson−Schottky equation derived from thermionic emission at 300 K, Jtherm = 1.08 × 107 exp(−ϕ/kT) exp(βE1/2), becomes dominant in determining the current injection.30 At a very low mobility, the injection current density is represented by J = n0eμE, producing the equation, Jmobility = 1.6μE exp(−ϕ/kT) exp(βE1/2). As the mobility increases, the pre-exponential term of the Richardson−Schottky equation becomes the limiting case. In a rough estimation based on the two equations, at a potential of 10 V for a film at a thickness of 100 nm, at least a mobility of ∼10

ality constant in the injection current being a function of mobility. In other words, the mobility term should be included in eq 2. Dependence of the net current on the carrier mobility in the injection-limited PbS/Au contact is validated by plotting the net current density J represented by PbS/Au contact resistance as a function of PbS channel mobility represented by PbS channel resistance in Figure 4. As noted earlier, in an injection-

Figure 4. Measurements of the contact and channel resistances in Au/ PbS/Au devices. (a) Plot of potential as a function of position from source electrode. For all samples, a drain voltage of 3 V was applied to measure the current and voltage drop at the source and drain electrodes to calculate the contact and channel resistances. The inset shows a schematic diagram of a device for the potential measurements. V1, V2, and V3 are spaced by 18 μm and L = 70, d = 36 μm, and Z = ∼2 mm. (b) Plot of contact resistance as a function of channel resistance. The thicknesses of PbS films in the devices were ∼100 nm, and the dimensions of the devices were same.

limited contact, the net current is dominated by the contact resistance, that is, Jnet ≈ Rc−1, justifying that the inverse contact resistance can represent the net current density in the injectionlimited contact. According to eq 1, the carrier mobility is inversely proportional to the channel resistance, Rch, that is, VD/ID ≈ μ−1/(Cox(VG−VT)), assuming that the carrier concentrations in PbS films, Cox(VG−VT), are comparable. Indeed, the threshold voltages, VT, for PbS NC films varied in a narrow range between −5 and 5 V, allowing the channel 6397

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Figure 5. Probing electronic and structural properties of PbS/Au contact using photocurrent microscopy (PCM). (a) Optical setup of PCM. (b) Photocurrent profile across the contact and channel regions of an EDT-treated PbS device. The thickness of the PbS film was ∼50 nm. The gold electrodes were not biased. (c) Photocurrent images in the PbS device. Photocurrent in the blue colored region in the channel is negligible. (d) Estimated energy band bending between Au/PbS/Au from the photocurrent profile in (b).

cm2/(V s) is required for thermionic emission current (Jtherm) to dominate over the mobility-dependent current injection (Jmobility), which is not easily achieved for most colloidal QD films. It is important to note that the energy barrier ϕ is included in the exponential function in both equations, indicating that the crossover at which mobility becomes a limiting parameter is independent of the magnitude of the energy barrier. It can be expected that, therefore, the choice of the metal electrode interfacing with QD films is not significant in determining the value of the mobility at the crossover. To summarize, in a device with an injection-limited contact, charge transport properties in the channel are more critical in determining charge injection than the injection energy barrier. We demonstrated that the net current density in Au/PbS/Au devices is dominated by contact properties through comparison between space charge limited current and a measured current. The net current is influenced by the channel mobility inversely proportional to the channel resistance according to the model where the net current can be decreased by back transfer of carriers accumulated near the metal electrodes due to a low mobility. PCM results clearly explain that a built-in electric field is present in PbS films, two-dimensionally continuous, near Au electrodes.

important because charge injection in an injection-limited contact is unavoidable in modern electronic devices, requiring a low operation voltage, including LEDs and photodetectors. Further, charge transfer is a crucial process governing device operation in QD devices. From our results, charge transfer should be understood not only in terms of energy level alignment, but also charge transport properties. To reduce the current loss at the metal/NC semiconductor, limitation based on energy barrier arising from work function of metal electrode can be mitigated by local doping at the interface, resulting in a reduced recombination current.

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EXPERIMENTAL SECTION

NC Synthesis and Characterizations. Lead oxide (PbO), oleic acid (OA), hexamethyldisilathiane (HMS), and 1-octadecence (ODE) were purchased from Sigma-Aldrich Co. Both hexane and methanol were obtained from Samchun Chemical. Twenty milliliters of OA was poured into a three-neck round-bottom flask with 0.45 g of PbO powder to make a mixture. The flask was evacuated for 5 min and then filled with nitrogen gas. This step was repeated three times to remove residual oxygen. The mixture was heated to 150 °C for 20 min under a nitrogen atmosphere until the mixture became a clear solution. This solution was cooled to 110 °C and was then evacuated again to remove any moisture. The solution then was reheated to 130 °C, becoming PbO-OA solution. TMS-ODE solution was prepared by dissolving 252 μL of HMS in 12 mL of ODE in an inert glovebox. Finally, 10 mL of TMS-ODE was injected rapidly into the above PbOOA solution at 130 °C and maintained at that temperature for 40 s to allow growth of PbS QDs. As soon as the reaction was completed, the reaction solution was cooled by immersion of the flask into an ice− water bath. Methanol then was added to the flask to precipitate crude PbS QDs. Centrifugation and washing with hexane/methanol was repeated twice to obtain purified PbS QDs. Device Fabrication and Electrical Characterization. To measure the contact and channel resistances, multiple voltage probes

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CONCLUSIONS Interestingly, the behavior of carriers in NCs is similar to that in organic materials with low mobility, evident from the dependence of net current on mobility in the channel in an injection-limited contact. More importantly, charge transport in PbS QDs dominated over the energy level difference between PbS and Au contacts in which hole injection is predicted to be favorable due to a low injection barrier. This finding is 6398

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were photolithographically patterned between the source and drain electrodes (Au/Cr) on a 200 nm SiO2 substrate. The voltage probes are spaced by 18 μm along the channel with a length of 70 μm. For field effect gating, a highly doped silicon substrate served as a gate electrode. Several drops of PbS NC solution in hexane were spincoated onto two-terminal devices at 1500 rpm for 60 s, producing a PbS thickness of ∼100 nm. After spin-coating of a PbS NC solution in hexane, solutions of ethanedithiol (EDT) or 4,4′-biphenyldithiol (BDT) in acetonitrile (ACN) were dropped onto a PbS film to replace OA with shorter ligands such as EDT and BDT, enhancing the electrical conductivity in the PbS films by increasing the tunneling current between PbS NCs.31 The PbS spin-coating procedure followed by ligand exchange with EDT or BDT solution was carried out multiple times to fill the voids in a PbS film resulting from decreased spacing between NCs after exchange of OA with shorter ligands.32 To calculate the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) levels of PbS NCs, UV−vis absorption spectroscopy and cyclic voltammetry (CV) were used. In CV measurements, the onset potential from PbS oxidation waves was used to calculate the PbS HOMO level, allowing for calculation of the hole injection barrier between Au and PbS. The ferrocene/ferrocinium (Fc/Fc+) redox couple was used to calibrate the measured onset potential from PbS NCs with respect to the vacuum level according to EHOMO = [(EOX − E1/2(ferrocene)) + 4.8 eV].33,34 For CV measurements, a platinum disk and platinum wire were used as the working and counter electrodes, respectively. To measure the potential relative to the electrodes, an Ag/AgCl electrode was used as a reference electrode. PbS NC solution in hexane was deposited onto the Pt electrode using drop-casting followed by ligand exchange by dipping the coated electrode into 0.1 M EDT solution in acetonitrile (ACN). After PbS film preparation, the working, counter, and reference electrodes were placed in an electrochemical cell filled with 0.1 M tetrabutylammonium perchlorate (TBAP) in ACN. The HOMO level of PbS NC was calibrated using 2 mM ferrocene in 0.1 M TBAP ACN. Varying the Channel Resistance. The channel resistance of twoterminal PbS devices was varied through thermal annealing,35 photoillumination,36 ligand exchange (EDT or BDT treatment),37,38 substrate surface treatment,39 and the use of different sized NCs.31,40,41 Thermal annealing was carried out in air at 100 °C for 2 h. For photoillumination, devices were illuminated with an incandescent light during I−V measurement. Two different ligands, EDT (0.1 M EDT in ACN) and BDT (0.1 M BDT in ACN), were used for ligand exchange, to replace OA with shorter ligands to vary the channel resistance, by increasing the current tunneling rate between NCs. The distance between NCs treated with EDT is shorter than that of BDT, and a lower channel resistance for EDT treated PbS films is anticipated. Additionally, the size of NCs used in our experiments was varied between 5.0 and 5.5 nm. For surface treatment, a poly(fluorine) polymer layer was spin-coated onto a SiO2 substrate. Photocurrent Microscopy. The local photocurrent near the PbS/ Au contact region was mapped using photocurrent microscopy (PCM). Optical excitation was carried out using a 785 nm laser. The local photocurrent was measured using an electrical setup consisting of a current preamplifier and a lock-in amplifier.42−44 A probe tip was scanned across the contact and channel regions at a constant scan speed. During the photocurrent scan, two-terminal PbS devices were not biased. The magnitude of the dark current at the PbS/Au contact, at a level of picoampere, increased to a nanoampere level upon illumination. The channel length and width of the device used for PCM measurement were 10 μm and 2 mm, respectively.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail:[email protected]. *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ○

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 2014 Hongik Faculty Research Support Fund. E.R. and M.C. acknowledge support from the Georgia Institute of Technology. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2014R1A1A1002636). H. Ko and D. Ko acknowledge the support by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF-2013M3C1A3065033).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Experimental conditions for fabrication of devices used in Figure 4. This material is available free of charge via the Internet at http://pubs.acs.org. 6399

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dx.doi.org/10.1021/cm502763z | Chem. Mater. 2014, 26, 6393−6400