I. INTRODUCTION
Two-dimensional (2D) layered semiconductors, especially transition metal dichalcogenides (TMDs), have emerged as exciting and versatile materials when their thickness is reduced to a monolayer or few-layers due to the emergence of quantum confinement effects and strong interfacial interactions. Reference Bulter, Hollen, Cao, Cui, Gupta, Gutiérrez, Heinz, Hong, Huang, Ismach, Johnston-Halperin, Kuno, Plashnitsa, Robinson, Ruoff, Salahuddin, Shan, Shi, Spencer, Terrones, Windl and Goldberger1,Reference Radisavljevic, Radenovic, Brivio, Giacometti and Kis2 Stacking different 2D semiconductors into van der Waals (vdW) heterojunctions creates a diverse palette of new, artificially-structured, layered materials with tunable optoelectronic properties depending on the stacking order, relative orientation angle, and atomic registry between the layers. Recently, p–n vdW heterojunctions have been constructed by either stacking or epitaxially growing different layered materials on top of one another or growing them laterally, and have demonstrated desirable optoelectronic functionalities such as photodetectors, photovoltaics, light-emitting diodes, and photodiodes. Reference Geim and Grigorieva3–Reference Withers, Del Pozo-Zamudio, Mishchenko, Rooney, Gholinia, Watanabe, Taniguchi, Haigh, Geim, Tartakovskii and Novoselov11 Persistent photoconductivity (PPC), an interesting optoelectronic effect where enhanced electrical conductivity persists after the removal of light illumination, Reference Shimakawa12–Reference Feng, Mönch, Harazim, Huang, Mei and Schmidt14 has so far received less attention, but has obvious potential for rewritable photoresponsive switches and memory elements. Reference Roy, Padmanabhan, Goswami, Sai, Ramalingam, Raghavan and Ghosh15
PPC has been primarily observed in compound semiconductors and layered structures. Reference Dang, Wang, Yu, Boutros and Redwing16–Reference Tebano, Fabbri, Pergolesi, Balestrino and Traversa18 However, 2D materials have suitable properties for PPC that include strong light-matter interaction, high carrier mobility, and gate tunability. Reference Roy, Padmanabhan, Goswami, Sai, Ramalingam, Raghavan and Ghosh15 While graphene has low photoresponsivity due to its intrinsic metallic property, hybrid graphene structures utilizing quantum dots or MoS2 atomic layers exhibit interesting PPC properties. Reference Roy, Padmanabhan, Goswami, Sai, Ramalingam, Raghavan and Ghosh15,Reference Konstantatos, Badioli, Gaudreau, Osmond, Bernechea, Garcia de Arquer, Gatti and Koppens19 TMDs, especially MoS2 and MoSe2, are highly photoresponsive materials with strong optical absorption and band gaps in the visible spectrum. vdW heterostructures constructed from these and other TMDs are good candidates for photoresponsive hybrid materials with charge exchange across the atomically sharp interfaces governed by quantum tunneling transport.
Doping or alloying have proven very effective ways to tune the optical and electrical properties in TMD monolayers. Reference Gong, Liu, Lupini, Shi, Lin, Najmaei, Lin, Elias, Berkdemir, You, Terrones, Terrones, Vajtai, Pantelides, Pennycook, Lou, Zhou and Ajayan20,Reference Suh, Park, Lin, Fu, Park, Jung, Chen, Ko, Jang, Sun, Sinclair, Chang, Tongay and Wu21 Recently, we found that isoelectronic substitution of Mo by W in the monolayer MoSe2 lattice can switch it from n-type to p-type behavior. Reference Li, Lin, Huang, Basile, Hus, Puretzky, Chen, Lee, Wang, Idrobo, Yoon, Li, Rouleau, Sumpter, Geohegan and Xiao22 The resulting alloy, i.e., Mo1−x W x Se2, has almost the same lattice constant as intrinsic MoSe2, but exhibits a tunable band gap with doping concentration, providing a tunable candidate for the construction of vdW heterojunctions.
In this study, vdW heterojunctions are fabricated by transferring and stacking monolayer single-crystals of MoSe2 and Mo1−x W x Se2 that were grown by vapor transport methods. The excellent heterojunction quality is assessed by aberration-corrected transmission electron microscopy (TEM), and strong interlayer coupling verified by low-frequency (LF) Raman spectroscopy and photoluminescence (PL). Electrical measurements are used to characterize the p–n junction formed by the stacked MoSe2 and Mo1−x W x Se2, which are also found to exhibit extremely high photoresponsivity and giant PPC at room temperature. Under time-dependent photo-illumination, these heterojunctions can function as rewritable optoelectronic switches or memory elements.
II. EXPERIMENTAL
A. Material growth
Crystalline monolayers of MoSe2 and Mo1−x W x Se2 were synthesized using a low pressure chemical vapor deposition (CVD) approach that is similar to those described previously. Reference Li, Duan, Wu, Zhuang, Zhou, Zhang, Zhu, Hu, Ren, Gao, Ma, Fan, Wang, Xu, Pan and Duan23 The synthesis was conducted in a tube furnace CVD reactor equipped with a 2 in. quartz tube. In a typical run, the growth substrates, i.e., Si wafer with 250 nm SiO2 (SiO2/Si) cleaned by acetone and isopropanol, were placed face down above an alumina crucible containing ∼0.2 g of MoO3 powder (for the growth of Mo1−x W x Se2, a mixture of MoO3 and WO3 powder was used), which was then inserted into the center of the quartz tube. Another crucible containing ∼1.2 g Se powder was located at the upstream side of the tube. After evacuating the tube to ∼5 × 10−3 Torr, flows of 40 sccm (standard cubic centimeter per minute) argon and 4 sccm hydrogen gas were introduced into the tube, and the reaction was conducted at 780 °C (with a temperature ramping rate of 30 °C/min) for 5 min at a reaction chamber pressure of 20 Torr. At 780 °C, the temperature at the location of Se powder was ∼290 °C. After growth, the furnace was cooled naturally to room temperature.
B. Monolayer crystal transfer and heterojunction fabrication
For the heterojunction fabrication and TEM sample preparation, poly(methyl methacrylate) (PMMA) was first spun onto the monolayer crystals on the SiO2/Si substrate at 3500 rpm for 60 s. The PMMA-coated substrate was then floated on 1 M KOH solution that etched silica epilayer, leaving the PMMA film with the monolayer crystals floating on the solution surface. The film was transferred to deionized water several times to remove residual KOH. For TEM samples, the washed film was captured on a Si TEM grid covered by a 50 nm-thick amorphous SiN film with 2 μm windows. For heterojunction fabrication, the film with the monolayer MoSe2 was stacked onto the substrate with the monolayer Mo1−x W x Se2. The PMMA was removed by acetone and baking at 300 °C in ∼30 Torr Ar/H2 (95%/5%) flowing for 2 h.
C. Device fabrication
Electron beam lithography (FEI DB-FIB with Raith pattern writing software; FEI Company, Hillsboro, Oregon) was used for monolayer MoSe2–Mo1−x W x Se2 heterojunction device fabrication. A layer of PMMA 495A4 was spun-coat on the SiO2 (250 nm)/Si substrate with monolayer MoSe2–Mo1−x W x Se2 heterojunctions, followed by a 180 °C annealing. After pattern writing, development, and lift off, a 10 nm layer of Ti followed by a 50 nm layer of Au was deposited using electron beam evaporation.
D. Characterizations
The morphologies of the monolayer MoSe2 and Mo1−x W x Se2 crystals were characterized using optical microscopy (Leica DM4500 P, Wetzlar, Germany), scanning electron microscopy (SEM, Zeiss Merlin, Oberkochen, Germany), and atomic force microscopy (AFM, Bruker Dimension Icon, Billerica, Massachusetts). The atomic structures of monolayer MoSe2 and Mo1−x W x Se2 were investigated via annular dark field (ADF) imaging using an aberration-corrected scanning transmission electron microscope (STEM) (ADF-STEM, Nion UltraSTEM™ 100, Kirkland, Washington) operating at 100 kV, using a half-angle of the ADF detector that ranged from 86 to 200 mrad.
Raman measurements were performed using a micro-Raman system (JobinYvon Horiba, T64000, Edison, New Jersey) based on a triple spectrometer equipped with three 1800 groves/mm gratings and a liquid nitrogen cooled charge-coupled device (CCD) detector. The Raman spectra were acquired under a microscope in backscattering configuration using 532 nm laser excitation (0.1 mW laser power). The excitation laser was focused to a ∼1 μm spot using a microscope objective (100x, numeric aperture, N/A = 0.9).
PL measurements were conducted using a home-built micro-PL setup, which included an upright microscope coupled to a spectrometer (Spectra Pro 2300i, Princeton Instruments, Acton, Massachusetts, f = 0.3 m, 150 grooves/mm grating) equipped with a CCD camera (Pixis 256BR, Princeton Instruments). The PL was collected through a 100x objective.
The electrical properties and photoresponse of the monolayer flakes and heterojunctions were measured in vacuum (∼10−6 Torr) under a probe station using a semiconductor analyzer (Keithley 4200, Keithley Instruments, Cleveland, Ohio) and a laser driven white light source with a power density of 64.42 mW/cm2 from 400 to 800 nm.
III. RESULTS AND DISCUSSION
Figure 1(a) shows an optical micrograph of the as-grown MoSe2 crystalline flakes and Fig. 1(b) shows a SEM image of the as-grown Mo1−x W x Se2 flakes on SiO2/Si substrates. The two types of flakes show the same triangular shape and similar sizes ranging from tens to a hundred micrometers. AFM images were used to measure the thickness of these crystals (∼0.7nm as in Fig. 1(c)), and correspond to monolayers of MoSe2 and Mo1−x W x Se2. This indicates that large-sized, uniform monolayer MoSe2 and Mo1−x W x Se2 were synthesized. To make MoSe2–Mo1−x W x Se2 heterojunctions, the as-grown monolayer MoSe2 flakes were transferred from the substrate and stacked onto the as-grown monolayer Mo1−x W x Se2 flakes. The optical micrograph [Fig. 1(d)] and SEM images [Figs. 1(e–f)] show that many stacked flakes are formed with arbitrary interlayer rotation angles, and the overlapping region can be clearly distinguished (as indicated by solid red arrows).
Morphologies of monolayer MoSe2, Mo1−x W x Se2, and stacked flakes. (a) Optical micrograph of the monolayer MoSe2 flakes grown on SiO2/Si substrate. (b) SEM image of the monolayer Mo1−x W x Se2 flakes grown on SiO2/Si substrate. (c) AFM image of monolayer Mo1−x W x Se2 flakes. Inset is the height profile along the solid blue arrow. (d–f) Optical micrograph and SEM images of as-grown monolayer MoSe2 flakes transferred and stacked onto as-grown monolayer Mo1−x W x Se2 flakes. The solid red arrows indicate the overlapping (junction) region.
Figure 2(a) shows a typical atomic resolution ADF-STEM (Z-contrast) image of an as-grown monolayer of Mo1−x W x Se2. A hexagonal lattice structure with a lattice constant a = 0.329 nm is clearly displayed (see also the fast Fourier transform [FFT] pattern in the inset), which is the same as pristine monolayer MoSe2. The atoms in Fig. 2(a) show different brightness since the intensity of the image is directly related to the atomic number of each atom. Figure 2(b) shows an intensity profile along the solid red arrow in Fig. 2(a), which typically represents the three types of sites in the monolayer Mo1−x W x Se2. The Se sites show slightly higher intensity than the Mo sites because each Se column contains two atoms, while the sites showing the highest intensity correspond to Mo substituted by W. Therefore, the monolayer Mo1−x W x Se2 shows alternating atomic arrangement of Mo and Se sites in the hexagonal rings, with the Mo atoms partially and randomly substituted by W atoms. According the ADF-STEM results, the local concentration of W is ∼18%. Figure 2(c) shows an ADF-STEM image of a monolayer of MoSe2/Mo1−x W x Se2 stack. Periodic Moiré patterns are observed due to interlayer rotation, and the FFT pattern [inset of Fig. 2(c)] shows that the two lattices are misaligned by ∼15°. The Moiré pattern also indicates that the interface between the two monolayers is clean and atomically sharp.
Atomic structure of monolayer Mo1−x W x Se2. (a) Atomic-resolution ADF-STEM image of monolayer Mo1−x W x Se2. Inset is the corresponding FFT pattern. (b) Intensity profiles along the solid red arrow in (a). (c) Atomic-resolution ADF-STEM image of the overlapping (junction) area of two stacked monolayer MoSe2 and Mo1−x W x Se2. Inset is the corresponding FFT pattern, indicating a 15° of interlayer rotation.
The optical properties of monolayer MoSe2, Mo1−x
W
x
Se2, and their stacked heterojunctions [Fig. 3(a)] were studied using Raman and PL spectroscopies at room temperature, using 532-nm laser excitation. Three typical Raman modes, i.e., the dominant A1g mode (out-of-plane), and the weak in-plane E1g and
${\rm{E}}_{2{\rm{g}}}^1$
modes, show up in the Raman spectra of both monolayer MoSe2 and Mo1−x
W
x
Se2 [Fig. 3(b)].
Reference Zhang, Wu, Zhu, Dumcenco, Hong, Mao, Deng, Chen, Yang, Jin, Chaki, Huang, Zhang and Xie24
The enlarged Raman spectrum [inset in Fig. 3(b)] indicates that the A1g mode of monolayer Mo1−x
W
x
Se2 is shifted from 240.9 (monolayer MoSe2) to 242.1 cm−1, which is consistent with previous reported monolayer Mo1−x
W
x
Se2 alloys.
Reference Zhang, Wu, Zhu, Dumcenco, Hong, Mao, Deng, Chen, Yang, Jin, Chaki, Huang, Zhang and Xie24
Figure 3(c) shows the room temperature PL spectra of individual monolayers of MoSe2 and Mo1−x
W
x
Se2. The as-grown monolayer of MoSe2 on SiO2/Si exhibits a single emission band peaking at ∼1.528 eV [Fig. 3(c), solid black curve], corresponding to radiative recombination of the A-exciton in monolayer MoSe2.
Reference Wang, Gong, Shi, Chow, Keyshar, Ye, Vajtai, Lou, Liu, Ringe, Tay and Ajayan25
The A-exciton emission in the as-grown monolayer of Mo1−x
W
x
Se2 on SiO2/Si shows a similar band shape, but exhibits a blue-shift of the emission peak to ∼1.554 eV [Figs. 3(c) and 3(d), solid red curves] compared with MoSe2. The blue shift can be related to the increase of band gap energy due to W-substitution. Interestingly, compared with the as-grown monolayer MoSe2 and Mo1−x
W
x
Se2, the transferred monolayers [e.g., the MoSe2 flake shown in Fig. 3(a)] both exhibits blue shift of the emission spectra, peaking at ∼1.577 eV [Figs. 3(c) and 3(d), solid green curve, obtained from the spot 1 in Fig. 3(a)] and ∼1.603 eV [dashed red curve in Fig. 3(c)], respectively. Such a blue shift can be attributed to the different interaction of the as-grown and transferred monolayers with the substrate.
Optical properties of monolayer MoSe2 and Mo1−x W x Se2. (a) Optical micrograph of CVD-grown monolayer MoSe2 (larger flake) transferred and stacked onto CVD-grown monolayer Mo1−x W x Se2 (smaller flake). (b) Raman spectra of the as-grown monolayer MoSe2 (black curve) and Mo1−x W x Se2 (red curve) with 532 nm laser excitation. Note that the spectra were offset for clarity. (c) Normalized PL spectra of the as-grown monolayer MoSe2 (solid black curve) and Mo1−x W x Se2 (solid red curve), and the CVD-grown monolayer MoSe2 (green curve) and Mo1−x W x Se2 (dashed red curve) after transfer with 532 nm laser excitation. (d) PL spectra obtained from spot 1 (transferred CVD-grown monolayer MoSe2), 2 (as-grown monolayer Mo1−x W x Se2), and 3 (overlapping region) labeled on (a), represented by solid green, red, and blue curves, respectively. (e) LF Raman spectrum (solid blue curve) obtained from the overlapping area in (a). The solid red curve is the calculated LF Raman spectrum.
To study the interlayer coupling in the heterojunction composed of MoSe2 and Mo1−x W x Se2, monolayers, we measured the PL from the overlapping region of the two flakes shown in Fig. 3(a). The PL intensity from the heterojunction [solid blue curve in Fig. 3(d)] is significantly quenched and the emission band is peaked at a lower energy (i.e., ∼1.507 eV) compared with the individual monolayer MoSe2 [solid green curve in Fig. 3(d)] and Mo1−x W x Se2 [solid red curve in Fig. 3(d)]. The significant PL quenching and red-shifted emission band suggest that the emission from the heterojunction area originates from interlayer charge recombination, Reference Fang, Battaglia, Carraro, Nemsak, Ozdol, Kang, Bechtel, Desai, Kronast, Unal, Conti, Conlon, Palsson, Martin, Minor, Fadley, Yablonovitch, Maboudian and Javey4 and the emergence of such emission also indicates strong interlayer coupling in the stacked heterojunction.
LF (below 50 cm−1) Raman spectroscopy contains rich information regarding the LF shear and breathing modes associated with in-plane and out-of-plane interlayer vibrations of the whole layers, which is an effective and sensitive way to understand the vdW interactions and coupling between layers in stacked 2D crystals. Reference Puretzky, Liang, Li, Xiao, Wang, Mahjouri-Samani, Basile, Idrobo, Sumpter, Meunier and Geohegan26 Figure 3(e) shows the LF Raman spectrum obtained from the stacked MoSe2/Mo1−x W x Se2 region in Fig. 3(a). Strong, narrow peaks associated with LF shear (at 19.1 cm−1) and breathing (at 32.9 cm−1) modes are observed, which is more evidence for strong interlayer coupling in the heterojunction. According to our previous study on bilayer MoSe2, when the two layers are stacked at exactly 60° interlayer rotation (i.e., 2H stacking configuration), the LF spectrum shows a dominant narrow shear mode peak at 19 cm−1 and a very weak, broad feature at ∼34 cm−1 related to the breathing mode. Reference Puretzky, Liang, Li, Xiao, Wang, Mahjouri-Samani, Basile, Idrobo, Sumpter, Meunier and Geohegan26 In the current stacked MoSe2/Mo1−x W x Se2 shown in Fig. 3(a), the interlayer rotation is measured to be 57.3°, slightly shifted from perfect 2H stacking, and this leads to significant enhancement of the breathing mode. Such a change in the LF Raman spectrum is attributed to the periodic arrangement of patches with different stacking configurations (i.e., high-symmetry 2H, AB′, A′B stacking and unaligned bilayers) as a result of the slight deviation in the interlayer rotation from 60° (i.e., perfect 2H stacking). Details of this study are presented elsewhere. Reference Puretzky, Liang, Li, Xiao, Sumpter, Meunier and Geohegan27
To investigate the optoelectronic properties of the MoSe2/Mo1−x W x Se2 heterojunction, a device was made by patterning source–drain contacts (Ti/Au) on both flakes while using the highly-doped Si substrate as the back-gating electrode [Fig. 4(a)]. Our previous study has already demonstrated that MoSe2 monolayers showed n-type conduction, which switched to p-type in Mo1−x W x Se2 monolayers. Reference Li, Lin, Huang, Basile, Hus, Puretzky, Chen, Lee, Wang, Idrobo, Yoon, Li, Rouleau, Sumpter, Geohegan and Xiao22 Indeed, the output (I ds–V ds) curve (at V bg = 0 V) from the MoSe2/Mo1−x W x Se2 heterojunction region clearly shows a typical rectifying behavior, with current only being able to pass through the device when the p-type Mo1−x W x Se2 is forward biased [Fig. 4(b), solid black curve]. To verify that the rectifying behavior originated from the junction, we also measured output curves of the individual MoSe2 and Mo1−x W x Se2 flakes in Fig. 4(a). The output curves show linear regions [inset of Fig. 4(c)], indicating Ohmic contacts between the electrodes and flakes. When shown on logarithmic scales, the output curves show very symmetrical behaviors [Fig. 4(c)], demonstrating that the rectifying behavior originated from the p–n heterojunction formed by stacking of Mo1−x W x Se2 and MoSe2 monolayers. The photoresponse of the heterojunction shown in Fig. 4(a) was studied using a white light lamp with power density of 64.42 mW/cm2 as the illumination source. Under white light illumination, the I ds (at V bg = 0 V) of the heterojunction was significantly increased by ∼3 order of magnitude compared to the dark current, with a photoresponsivity of ∼13.2 A/W at V ds = −5 V [Fig. 4(b), solid red curve]. This result demonstrates that the stacked heterojunction have a very good photoresponse. However, after removal of light source, the current in the p–n junction did not immediately decrease back to that originally measured for the dark state; instead, it experienced a fast decay in the first hour and then decreased very slowly, persisting with enhanced photocurrent for more than 2 days [Fig. 4(d)]. The higher conductivity state activated by light exposure persisted for a long time (several days) after the removal of light, showing a giant PPC. The current decay can be fitted exponentially, with a time constant τ of ∼12.2 h [Fig. 4(e)]. Interestingly, the PPC was not observed from single monolayers of MoSe2 or Mo1−x W x Se2, which show fast response time as the light is switched on and off [Fig. 4(f)], typical for the photoresponse behavior of 2D TMDs. It can be concluded that the giant PPC comes only from the junction area. The giant PPC in the monolayer MoSe2–Mo1−x W x Se2 heterojunction may be due to the following two reasons: (i) the existence of potential barriers or surface barriers on the interface of the heterojunction that spatially separate the photo-generated charge carriers. Reference Tebano, Fabbri, Pergolesi, Balestrino and Traversa18 (ii) The possible existence of traps at the interface of the heterojunction that extend electron-hole recombination lifetime. The detailed mechanism behind the PPC behavior is under further study.
PPC and photoresponsive switching of the MoSe2–Mo1−x W x Se2 heterojunction. (a) Optical micrograph of a CVD-grown monolayer MoSe2 (upper flake) transferred and stacked onto a CVD-grown monolayer Mo1−x W x Se2 (lower flake). The overlapping region of the two flakes forms a heterojunction. Electrodes 1 and 2 were patterned on MoSe2 while 3 and 4 on Mo1−x W x Se2. (b) I ds–V ds (with I ds shown in absolute values) curves of the heterojunction area shown in (a) on logarithmic scales at zero back-gate voltage in dark (solid black curve) and under white light illumination (solid red curve). (c) I ds–V ds (with I ds shown in absolute values) curves of the individual monolayer MoSe2 (solid blue curve) and Mo1−x W x Se2 (solid red curve) shown in (a) on logarithmic scales at zero back-gate voltage in dark. Inset is the corresponding I ds–V ds curves on linear scale. (d) I ds–V ds curves of the heterojunction at zero back-gate voltage in dark (solid black curve), under white light illumination (solid red curve), and after removal of the illumination for 5 min (solid blue curve), 10 min (solid orange curve), 20 min (solid green curve), 30 min (solid magenta curve), 40 min (solid yellow curve), and 3000 min (solid pink curve). (e) Decay of the photocurrent after removal of white light source (solid squares), which was fitted exponentially (red curve) with τ = 12.2 h. (f) Time-resolved photoresponse at a bias voltage of −5 V of the individual Mo1−x W x Se2 flake shown in (a). (g) Photocurrent induction and switching operation of the heterojunction. The gray-shaded areas indicate the presence of white light illumination, during which the current gradually increased to saturation level at V ds = −5 V. The green dashed vertical lines indicate the application of gate pulses (100 V), where the current was recovered to the initial level.
The photoresponse of the MoSe2–Mo1−x W x Se2 heterojunction was further investigated by studying time-resolved current change at V ds = −5 V. Figure 4(g) shows the result of three photo-illumination cycles. Upon illumination, the photocurrent (−I p) increased gradually and saturated at a certain level after ∼120 s (at zero back-gate voltage). After switching the light off, the device showed PPC with slow decay of current (with a time constant τ of ∼12.2 h); however, the current could be rapidly and fully recovered to the initial level (dark state) by applying a gate bias pulse at 100 V, which equilibrated the charge distribution. Repeating these steps again resulted in the same behavior, showing the robustness of this heterojunction. Such behavior indicates that the MoSe2–Mo1−x W x Se2 heterojunction with PPC can potentially be designed to function as photoresponsive memory devices.
In summary, vdW heterostructures were fabricated by transferring and stacking as-grown crystalline monolayers of MoSe2 onto as-grown monolayers of Mo1−x W x Se2. These heterostructures showed strong interlayer coupling as proven by the emergence of interlayer charge transfer PL and the presence of LF Raman modes. When illuminated with white light, the heterojunction shows extremely high and persistent photocurrent lasting days, which appear applicable for rewritable optoelectronic switch and memory applications involving time-dependent photo-illumination. This MoSe2–Mo1−x W x Se2 heterojunction can potentially lead to new monolayer TMDs-based optoelectronic devices.
ACKNOWLEDGMENTS
Synthesis science sponsored by the Materials Science and Engineering Division, Office of Basic Energy Sciences, U.S. Department of Energy. Materials characterization conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. L.B. acknowledges the financial support of the National Secretariat of Higher Education, Science, Technology and Innovation of Ecuador (SENESCYT). X.L. and M.L. acknowledge support from ORNL Laboratory Directed Research and Development.
