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Manipulation of Optoelectronic Properties and Band Structure Engineering of Ultrathin Te Nanowires by Chemical Adsorption Ahin Roy,†,‡ Kazi Rafsanjani Amin,†,§ Shalini Tripathi,‡ Sangram Biswas,§ Abhishek K. Singh,‡ Aveek Bid,*,§ and N. Ravishankar*,‡ ‡
Materials Research Centre and §Department of Physics, Indian Institute of Science, Bangalore 560012, India S Supporting Information *
ABSTRACT: Band structure engineering is a powerful technique both for the design of new semiconductor materials and for imparting new functionalities to existing ones. In this article, we present a novel and versatile technique to achieve this by surface adsorption on low dimensional systems. As a specific example, we demonstrate, through detailed experiments and ab initio simulations, the controlled modification of band structure in ultrathin Te nanowires due to NO2 adsorption. Measurements of the temperature dependence of resistivity of single ultrathin Te nanowire field-effect transistor (FET) devices exposed to increasing amounts of NO2 reveal a gradual transition from a semiconducting to a metallic state. Gradual quenching of vibrational Raman modes of Te with increasing concentration of NO2 supports the appearance of a metallic state in NO2 adsorbed Te. Ab initio simulations attribute these observations to the appearance of midgap states in NO2 adsorbed Te nanowires. Our results provide fundamental insights into the effects of ambient on the electronic structures of low-dimensional materials and can be exploited for designing novel chemical sensors. KEYWORDS: tellurium, nanowire, adsorption, band-structure engineering, S-M transition, electronic transport transition in n-type mesoporous Si owing to NH3 adsorption.18 In this letter, we illustrate such a scenatio for ultrathin Te nanowires (TeNW), which undergo S-M transition upon NO2 adsorption. In terms of the nanoscale Te, it has been studied extensively for various applicative aspects. Te thin-films,19 thicker Te nanowires and whiskers20 and Te nanowire-based composites21 have been studied for thermoelectric applications,22 electrical transport,23 colorimetric sensing of Hg,24 and selective sensing of NOx-based gases.25−30 Te nanowires have been used as templates to fabricate other one-dimensional nanostructures31 and Te nanowire based heterostructures32 have been utilized for flexible electronics. Surprisingly, in spite of these attractive properties, electronic transport in single TeNW-based device has never been addressed in detail before. While the semiconducting nature of these nanowires opens up the possibility of utilizing them for sensing applications, their high carrier mobility and ultralow band gap make them promising candidates for IR-detection. Furthermore, as these nanowires are grown along the crystallographically anisotropic c-direction of the bulk Te, the unidirectional conduction in these wires is free of any grain-boundary scattering resulting in very low intrinsic noise levels and a very high barrier to
1. INTRODUCTION In terms of properties and electronic structure, materials display significantly different behavior at the nanoscale as compared to their bulk counterparts. Needless to mention, to achieve a control to fine-tune the materials properties, a thorough understanding of novel phenomena at nanoscale is indispensable. One such aspect is semiconductor to metal (S-M) transition observed in various materials caused by a variety of external or internal perturbations. For example, ZnO is known to undergo an S-M transition upon Ti doping.1 Various transition metal dichalcogenides undergo an S-M transition under hydrostatic pressure.2 WO3 shows a metallic behavior in the presence of O-vacancies.3 Carbon nanotubes show such behavior under electric field,4 ferroelectric VO2 shows mesoscopic metal−insulator transition at the domain walls,5 and layered black phosphorus exhibits S-M transition with liquid gating.6 Semiconducting InGaZnO7 and layered boron nitride undergo S-M transition upon hydrogenation.8 Several materials undergo pressure-induced S-M transition,9−17 wherein the pressure induces either a complete phase change,9,10,13 or a change in valency11/electronic distribution12 of the transition metal, leading to an interaction of charge-density on the cation and the anion,17 resulting in the emergence of new states at the Fermi level. As evident from the above examples, all the S-M transitions involve severe chemical transformation or physical changes owing to pressure. Small surface perturbation induced S-M transition is very rare in literature. To the best of our knowledge, there is only one report which demonstrates S-M © XXXX American Chemical Society
Special Issue: Focus on India Received: September 22, 2016 Accepted: December 30, 2016
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DOI: 10.1021/acsami.6b12064 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 1. (a) X-ray diffraction pattern of as-synthesized TeNW, the bottom panel showing the phase-pure trigonal crystal structure; (b) Lowmagnification transmission electron micrograph showing bundles of TeNWs formed in the reaction. (c) High-magnification TEM image of TeNW. (d) Diameter-distribution histogram showing the ultrathin nature of the nanowires; the distribution shows that a large number of wires have ∼5 nm diameter; (e) electron diffraction pattern obtained from a single nanowire; (f) HRTEM of TeNW showing the marked planes of Te. TeNW using density functional theory (DFT) in conjunction with allelectron projector augmented wave potentials33,34 and the Perdew− Burke−Ernzerhof35 generalized gradient approximation to the electronic exchange and correlation, as implemented in the Vienna Ab initio Simulation Package (VASP). To model the experimentally synthesized TeNW, the smallest possible unit cell of a TeNW was cleaved from [0001]-oriented bulk Te with a hexagonal prism morphology. Crystallographically, the wire is bound by {112̅0} type facets. More than 12 Å vacuum was added in the transverse directions of the unit cell to avoid interactions between periodic replicas of the nanowire. A well-converged Monkhorst−Pack k-point set of 1 × 1× 5 was used for the calculations. Conjugate gradient scheme was employed to optimize the geometries along the periodic direction of the nanowire, until the forces on each atom were ≤0.005 eV/Å.
electromigration damage. Thus, these nanowires provide an ideal platform for studying the effect of external perturbation on the unidirectional conduction in an ultralow band gap semiconductor. In this letter, we demonstrate of molecular-doping induced semiconductor to metal transition in ultrathin single-crystalline trigonal TeNW. In a coupled approach involving experiments and ab initio simulations, we have studied the effect of NO2 exposure on the electrical transport behavior of singlecrystalline TeNW and demonstrate a novel semiconductor to metal transition upon adsorption of NO2. These wires, synthesized via a polyol-assisted hydrothermal method, interact electronically with the NO2, leading to a decrease in their resistance upon adsorption. The temperature dependence of the resistance of the pristine and the NO2 exposed TeNW clearly illustrate the fundamental differences in the electronic structure of the two systems. Our simulations show that the SM transition occurs because of the introduction of a new band near Fermi energy upon adsorption of NO2. Furthermore, the calculated electronic density of states (DOS) illustrate the hybridization of the Te p-orbitals with the analyte, leading to the emergence of new energy states at Fermi level, which were absent in the bare nanowires. Raman shifts of the nanowire prior to and after the exposure of the NO2 exhibit a huge contrast, illustrating the essential difference between the two systems.
3. RESULTS AND DISCUSSION The Te nanowires used in the study were synthesized via a wetchemical route based on the following reaction, wherein hydrazine (N2H4) acts as a reducing agent, reducing Te4+ in the Te-precursor to elemental Te TeO32 − + N2H4 = Te + 2OH− + H 2O + N2
(1)
Tellurium, being trigonal in the crystal structure, has 31 screw axis along the c-axis. This results in an inherent anisotropy in the bulk structure along the c-axis. Consequently, Te prefers to grow in an one-dimensional morphology along this direction. PVP used in the reaction acts as a diameter controlling capping agent, restricting the radial growth of the nanowires. Furthermore, it has been shown in a previous report36 that an optimal pH of 12 of the initial solution is crucial for the formation of the ultrathin nanowires. Although a lower pH (pH 8) gives rise to thicker nanorods, a higher pH (pH 14) leads to the formation of nanoparticles. The NH3 is used to maintain the optimum pH during the synthesis. As can be seen from the X-ray diffraction pattern in Figure 1a, and the morphology in the bright-field TEM image in Figure 1b, phase pure Te nanowires were formed by the controlled reaction. These nanowires were ultrathin (∼2−5 nm diameter) as can be seen from the diameter distribution histogram Figure 1d. The amount of the capping agent, namely PVP in the reaction medium, and the reaction time are the key
2. EXPERIMENTAL AND SIMULATION METHODS 2.1. Wet Chemical Synthesis Procedure. The wet-chemical synthesis of ultrathin Te nanowires was carried out through a hydrothermal method reducing Na2TeO3. In a typical synthesis, 92 mg of Na2TeO3 and 1.0 g PVP (molecular weight = 55 000) were dissolved in 35 mL of water followed by the addition of 1.65 mL hydrazine hydrate (N2H4. H2O) and 3.35 mL of NH3 solution. The whole solution was transferred to a Teflon lined vessel, sealed in an autoclave and heated at 180o C for 4h. The resultant black product was extracted with acetone followed by vigorous washing with warm DI H2O. The product was never subjected to sonication to avoid breaking of the nanowires. 2.2. DFT Calculation Methodology. To support the observations, we calculated the total energy and the electronic structure of B
DOI: 10.1021/acsami.6b12064 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. (a) False color bird-eye view SEM image of a typical single nanowire device. The nanowire is shown in green and the electrical contacts are shown in violet. PA represents voltage preamplifier and LIA represents lock-in amplifier.) (b) Source−drain current as a function of bias-voltage of a typical pristine TeNW device, the linearity of the plot shows that good ohmic contacts were achieved. (c) Resistance R of a single pristine TeNW as a function of the back gate voltage Vg, measured at room temperature. The resistance R increased with increase in Vg indicating that the majority carriers in the system were holes. (d) Resistance versus temperature of a pristine TeNW device; increase in resistance with decreasing temperature indicates a semiconducting behavior of the pristine nanowire.
Figure 3. (a) Plot of the % change in the resistance R of a single TeNW with exposure to NO2. The instants of exposure to NO2 and pumping out of NO2 from the measurement chamber are indicated by arrows. Negligible change in the R even after prolonged evacuation of the chamber, indicates surface adsorption of NO2 to the TeNW. (b) Transition from semiconducting to metallic regime of TeNW nanowire with increasing concentration of NO2. The legend commonly applies to plots b−f. (c) Plots of ln(R) vs 1/T in the high temperature thermally activated transport regime, for measurements with exposures to different concentrations of NO2. The thick lines are linear fits to the data. (d) Plots of ln (RT−0.8) vs T−0.25 in the low-temperature regime. Linearity of the data indicates that the electrical transport in this regime is determined by Mott variable range hopping. Thick lines are linear fits to the data. (e) Plot of activation energy Eg, evaluated from slopes of plots in c as a function of NO2 concentration. (f) Plot of Mott temperature (T0), evaluated from slopes of plots in d as a function of NO2 concentration.
factors controlling the radial growth of the nanowire. Presence of a very large quantity of PVP compared to Te-precursor and very small reaction time (4 h) lead to formation of ultrathin wires37−41 in our case. The electron diffraction (Figure 1e) from the wires confirms the trigonal phase of the Te and HRTEM shows the {101̅1} planes corresponding to Te. The removal of capping agent PVP is ensured through the vigorous cleaning. As can be seen from XPS data shown in Figure S1, the Te0/Te2+ ratio increases significantly upon cleaning of the nanowires. The nanowires after synthesis were drop casted on a Si/SiO2 wafer having predefined alignment markers. Single isolated nanowires were identified by optical microscope. Scanning electron microscopy (SEM) could also be used to identify ultrathin single nanowires, although, in order to avoid surface contamination of nanowires, SEM imaging before measurement was avoided. Electrical contacts to the single nanowire devices
were defined by standard electron beam lithography techniques followed by thermal metallization of Cr/Au or Ti/Au (5/60 nm). Low-power Ar plasma treatment was carried out just before the metal evaporation, to remove polymer residues and oxides from surface of nanowires. We observed that this plasma treatment was crucial for obtaining low-resistance contacts on the TeNW. The doped Si substrate was used as the back-gate contact with the 295 nm thick SiO2 acting as the gate dielectric layer for the field effect transistor devices. Figure 2a shows the falsecolored SEM image of a typical single-nanowire device. Resistance of the single TeNW devices were measured using standard low frequency lock-in detection techniques in a 4probe configuration. A schematic of the measurement scheme is shown in Figure 2a. Figure 2b shows a plot of the sourcedrain current as a function of biasing source−drain voltage. The linearity of the curve indicates the ohmic nature of the contacts. C
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Figure 4. Raman spectra of two different Te nanowires upon exposure to increasingly higher concentrations of NO2; the volume percentage of NO2 in buffer of dry N2 gas is mentioned in the legend. The intensity of the Raman peaks decreased monotonically and finally vanished as the TeNW were exposed to higher amounts of NO2.
We observed that there were two distinct regimes of transport for the TeNW devices at low NO2 induced doping levels. In the high temperature (190 K - 280 K) regime the transport followed a thermally activated Arrhenius behavior:
Furthermore, the contact resistance were estimated from the difference between two-probe and four-probe resistances, and was found to be very small as compared to the nanowire channel resistance. Figure 2c shows the back-gate voltage (Vg) dependence of the channel resistance of a typical device measured in a 4-probe configuration. The resistance monotonously decreased with increase in magnitude of negative Vg indicating that the majority charge carriers in the system were holes, which was in conformity with previous reports.42 As shown in Figure 2d, the resistance of the pristine nanowire device was found to increase monotonically with decreasing temperature for all values of Vg. This showed that the TeNW retained the low band gap semiconducting nature of bulk tellurium. To test the effect of exposure to NO2 gas, we placed the TeNW device in an evacuated chamber and controlled amounts of NO2 gas was introduced into the chamber. The resistance of the TeNW was found to sharply decrease and then equilibrate at a much smaller value. Figure 3a shows a plot of the resistance of a single TeNW as a function of time during the of exposure to NO2. We observed that the resistance of the device did not reset to its initial value even upon prolonged pumping of the vacuum chamber, indicating a strong binding between the NO2 and the TeNW. To investigate in detail the effect of NO2 adsorption on the TeNW, we measured the temperature dependence of the resistance R after exposure to increasingly higher amounts of NO2. Figure 3b−f summarizes the results of the measurements on a particular device. In this measurement, the device was mounted in a variable temperature cryostat and exposed to a certain concentration of NO2 in a buffer of pure nitrogen gas. After the resistance of the device saturated, the cryostat was evacuated for an extended period of time to pump out the unadsorbed NO2 gas before measuring the temperature dependence of the resistance of the device. This process was repeated with increasing concentrations of NO2. With increasing amounts of NO2, we could observe a gradual, but clear transition of the nanowire from a semiconducting state (characterized by dR/dT < 0) to a metallic state (characterized by dR/dT > 0) (Figure 3b). Similar result of semiconducting to metal transition have been observed in nanowire of relatively thicker diameter (∼40 nm) (data shown in Figure S2). However, the effect, as quantified by the fractonal change in resistance due to adsorption of NO2 is much stronger in ultrathin nanowires.
R(T ) = R 0exp[Eg /kBT ]
(2)
where Eg is the activation energy. Figure 3c shows plots of ln(R) vs 1/T for exposure of the TeNW to different concentrations of NO2 - the solid lines are linear fits to the data using eq 2. The activation energy Eg extracted from the slope of linear fit to data is plotted in Figure 3e. With exposure to increasing concentration of NO2, Eg decreases and eventually becomes zero as the nanowire becomes metallic. We find that the value of the activation energy is quite small even for the pristine nanowire. This is consistent with previous observations for high-quality single-crystal tellurium samples43 and can be explained using the two-carrier model introduced by Takita et al.,44,45 where it is proposed that there exists impurity bandlike states that can arise either from the short-range potential caused by lattice defects or from isovalent impurities.46 In the low-temperature regime, the electrical transport in the TeNW device changes from thermally activated to variable range hopping. We found that over the temperature range of 90 to 175 K, the data could be well-fitted to a Mott variable range hopping (VRH)47 R(T ) = R1(T )exp[(T0/T )1/(d + 1)]
(3)
where d is the dimension of the system. For all the TeNW studied and at all NO2 doping ranges in the semiconducting regime, the data could be fitted best with d = 3 with a powerlaw temperature dependence of R1(T) ∝Tm, m ≈ 0.8−1.0. This can be understood from the fact that the estimated coherence length of Te is smaller than the diameter d of the TeNW measured by us and hence the TeNW are effectively in the 3dimension limit. Figure 3d shows the fit to the resistivity data in the low temperature regime using eq 3 and the extracted values of T0 as a function of NO2 concentration are plotted in Figure 3f. The Mott characteristic temperature T0 is related to localization length ξ and DOS at Fermi energy N(EF) as48 kBT0 = [18/N (E F)ξ 3]
(4)
As the semiconducting to metal transition is approached from the semiconducting side, ξ diverges. Consequently, T0 goes smoothly to zero at the S-M transition, as can be seen from Figure 3f. D
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elastic properties have shown that such nanowires are stable and that their elastic moduli are comparable to that of bulk Te.53 Electronically, in its bulk state, Te is a semiconductor with a band gap of 0.25 eV at H-point.22 Upon nanostructuring, the band gap of the nanowire is enhanced to 0.65 eV at X-point, as can be seen from the calculated electronic band structure and DOS of the nanowire (Figure 5). The reason for the enhancement can be attributed to the quantum confinement arising because of down-sizing of Te from its bulk state. Such enhancement in band gap for Te nanowire has been reported before.53 Being an ambidentate chemical moiety, NO2 can bind to Te nanowire through either N or O. We have carried out DFT simulations to study the two adsorption geometries of NO2, as shown in Figure 6a, d. The adsorption geometries were constructed using edge site of TeNW as the docking site for the NO2 molecule. The binding energy (Eb) of the NO2 on the TeNW can be defined as
For the Mott VRH model to be applicable in a system, the following condition needs to be rigorously satisfied48,49 Δhop > 2ΔCG
(5)
where Δhop is the Mott hopping energy and ΔCG is the Coulomb gap in the electronic DOS. This in turn leads to the criterion that for Mott VRH to occur the temperature T must satisfy the condition48 T > T0/465
(6)
From the values of T0 obtained from the fits to the temperature dependence of resistance data, we estimate that Mott VRH should be valid in our TeNW devices for temperatures above 50 K. Subsequently, we have also studied the effect of doping on the Raman vibrational modes of ultrathin Te nanowires. The data are shown in Figure 4 for two different TeNW samples. As the growth direction of the TeNW was along the trigonal axis, the bulk of the Raman signal was obtained for phonon propagation directions perpendicular to this direction. Hence, the E modes are split into transverse ETO and longitudinal ELO branches. The position of these modes as well as that of the of the A1 mode matched very well with earlier reports.50−52 The nanowires were then exposed to high concentration of NO2, and the Raman spectra were recorded. Figure 4 shows the Raman spectra of the NO2 exposed nanowires. It can be seen that intensity of the Raman peaks gradually reduce with exposure to increasing amounts of NO2 and ultimately disappear. A material is Raman active when vibrational modes can result in a change in its polarization. Metallic systems are infinitely polarizable because of the screening by free charge carriers. Hence vibrational modes can not cause change in polarizability making metallic systems Raman inactive in general, although some residual Raman activity can still come from electronic modes. The quenching of the vibrational Raman peaks of pristine Te after exposure to NO2 indicates a significant increase in the free charge carrier density in the system and is in clear agreement with a transition from semiconducting state of pristine TeNW to metallic state of NO2 adsorbed TeNW. To gain further insight into the S-M transition, we carried out electronic structure study of pristine and NO2 adsorbed nanowires. To model the ultrathin TeNW, [0001] (c-axis) oriented TeNW were cleaved from bulk Te. Crystallographically trigonal TeNW is composed of triangular building blocks, each vertex of the triangle being located in a different plane along the 31 screw axis (Figure 5). Thus, going along the c-axis of bulk Te, we get an ABC type packing. The modeled nanowire has a diameter of 1.8 nm and consists of concentric hexagons, as shown in Figure 5. Investigations of the basic
E b = E TeNW + NO2 − E TeNW − E NO2
(7)
where ETeNW+NO2 is the energy of the NO2 adsorbed TeNW, ETeNW is the energy of the bare TeNW, and ENO2 is the energy of the molecular NO2 in its unbound state. While binding through nitrogen, NO2 was found to have an Eb of −0.53 eV with a Te−N bond length of 2.8 Å. On the other hand, when NO2 is adsorbed on TeNW through O atom, the Eb is −0.56 V and the Te−O bond length is 2.2 Å. Thus, the calculations show that the binding of NO2 is energetically slightly favorable for binding through O atom. The high binding energy of NO2 indicates toward a possibility of chemisorption of the NO2 molecule on the TeNW, which is expected to render a strong effect on the electronic structure of the system. As can be seen from the band structure of the NO2 adsorbed nanowire (Figure 6b), the electronic structure of the Te nanowire is indeed modified because of adsorption. We observe an appearance of a midgap state across the Fermi energy in the band structure, resulting in an adsorption-induced semiconductor to metal transition. Furthermore, electronic DOS calculation shows that there is an increase in the total number of states at the Fermi energy, as depicted in Figure 6c. To gain further insight into the adsorption-induced electronic effects, ldecomposed adsorption site projected DOS (PDOS) was calculated. The PDOS was calculated by projecting the electronic wave function of the system onto the spherical harmonics of the sphere around the Te atom on which the NO2 was adsorbed. The PDOS showed that there was an increase in the number of p-states of Te upon adsorption, which arose because of the hybridization of the NO2 molecule with the nanowire. The increase of DOS (N(EF)) at Fermi level of TeNW with NO2 doping was also in an excellent agreement with the transport measurements, where a decrease in Mott temperature T0 (Figure 3f) was observed as the system underwent an S-M transition with increasing NO2 doping. It should be noted that NO2 is a widely known electron aceptor, and hence the charge transfer from NO2 to Te nanowire is counterintuitive. However, Te has a higher electron affinity (190.2 kJ mol−1) than that of NO2 (174.33 kJ mol−1),54 resulting in an acceptance of electrons from NO2 upon adsorption, in turn causing the S-M transition, which manifests as a decrease in the device resistance. Usually when an analyte adsorbs on a material, the electrical conductivity of the system can get modified due to two reasons:
Figure 5. (a) Atomistic view of the Te nanowire with a hexagonal cross-section, consisting of two concentric hexagons; (b) electronic band structure density of states of the nanowire. E
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Figure 6. (a) Adsorption geometry of NO2 on Te nanowire with N as the docking site; (b) band structure of the nanowire after NO2 adsorption, and (c) change in electronic DOS of the nanowire due to adsorption - upper panel shows the p-decomposed adsorption site projected DOS, and the lower panel shows total DOS. (d) adsorption geometry of NO2 on Te nanowire with O as the docking site; (e) band structure of the nanowire after NO2 adsorption and (f) change in electronic DOS of the nanowire due to adsorption; upper panel shows the p-decomposed adsorption site projected DOS, and lower panel shows total DOS. The enhancement of the DOS at the Fermi level has been marked by arrows.
devices. Our study provides a novel technique to achieve this by surface adsorption of appropriate chemicals at room temperature. This opens up a platform to utilize a wide range of properties of semiconducting as well as metallic states of the system.
(i) adsorption-induced carrier scattering, which always decreases the conductivity, and (ii) adsorption-induced electronic structure modification of the substrate. In the case of TeNW, because of the strong hybridization between the nanowire and NO2, the electronic effect dominates and in this particular case this leads to a decrease in the resistance of the device which we have been able to capture experimentally. It has been hypothesized before that upon NO2 adsorption on the Te film, a trapped charge state is created on the Te surface,55 leading to the chemical interaction of Te films with NO2. We show that for Te nanowires, the interaction is much more direct and leads to appearance of a midgap state. Interestingly, the SM transition occurs for both the adsorption scenarios, indicating the strong ambidentate nature of NO2 toward Te.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12064. XPS studies of Te nanowires before and after cleaning, semiconductor to metal transition on thick Te nanowires (PDF)
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4. SUMMARY In summary, we have reported a novel surface adsorption induced technique for band gap engineering of nanostructured semiconducting materials. We demonstrated this by specifically studying the semiconductor-metal transition in single-crystalline ultrathin Te nanowires upon “molecular-doping” with NO2. Our study sheds light on the mechanistic understanding on the much-researched NO2 adsorption on Te nanowire. The observed S-M transition was related to an decrease in the activation energy and an increase of localization length ξ with increasing surface adsorption of NO2. On the basis of ab initio simulations, this phenomenon was understood on the basis of the strong hybridization of the Te p-states with the adsorbed NO2, leading to the appearance of midgap states. The molecular doping by NO2 not only changes the nature of charge transport in the nanowires, but also its vibrational phonon modes as observed from Raman spectra. Band structure engineering in semiconductor materials often provides significant performance increase in existing materials and imparts new functionalities to electronic and optoelectronic
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected] ORCID
Aveek Bid: 0000-0002-2378-7980 N. Ravishankar: 0000-0003-0012-046X Author Contributions †
A.R. and K.R.A. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS K.R.A and S.T. thank CSIR, MGRDG, Govt. of India, for financial support. A.B. and A.K.S. acknowledge funding from Nanomission, Department of Science & Technology (DST), Govt. of India, and HRDG, CSIR, Govt. of India. A.R. and A.K.S. thank the Supercomputer Education and Research Center (SERC) and Materials Research Centre (MRC), IISc, F
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(19) Damodara Das, V.; Jayaprakash, N.; Soundararajan, N. Thermoelectric power of tellurium thin films and its thickness and temperature dependence. J. Mater. Sci. 1981, 16, 3331−3334. (20) Bodiul, P.; Bondarchuk, N.; Huber, T.; Konopko, L.; Nikolaeva, A.; Botnari, O. Thermoelectric Properties of Films and Monocrystalline Whiskers of Tellurium. In 25th International Conference on Thermoelectrics; IEEE: Piscataway, NJ, 2006; pp 607−610. (21) Coates, N. E.; Yee, S. K.; McCulloch, B.; See, K. C.; Majumdar, A.; Segalman, R. A.; Urban, J. J. Effect of Interfacial Properties on Polymer-Nanocrystal Thermoelectric Transport. Adv. Mater. 2013, 25, 1629−1633. (22) Peng, H.; Kioussis, N.; Snyder, G. J. Elemental tellurium as a chiral p-type thermoelectric material. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 195206. (23) Liang, F.; Qian, H. Synthesis of tellurium nanowires and their transport property. Mater. Chem. Phys. 2009, 113, 523−526. (24) Wei, T.-Y.; Chang, H.-Y.; Lee, Y.-F.; Hunga, Y.-L.; Huang, C.-C. Selective Tellurium Nanowire-based Sensors for Mercury(II) in Aqueous Solution. J. Chin. Chem. Soc. 2011, 58, 732−738. (25) Tsiulyanu, D.; Moraru, A. Nanoscience Advances in CBRN Agents Detection, Information and Energy Security; Springer: New York, 2015; pp 389−408. (26) Wang, Z.; Wang, L.; Huang, J.; Wang, H.; Pan, L.; Wei, X. Formation of single-crystal tellurium nanowires and nanotubes via hydrothermal recrystallization and their gas sensing properties at room temperature. J. Mater. Chem. 2010, 20, 2457−2463. (27) Park, H.; Jung, H.; Zhang, M.; Chang, C. H.; Ndifor-Angwafor, N. G.; Choa, Y.; Myung, N. V. Branched tellurium hollow nanofibers by galvanic displacement reaction and their sensing performance toward nitrogen dioxide. Nanoscale 2013, 5, 3058−3062. (28) Tsiulyanu, D.; Marian, S.; Miron, V.; Liess, H.-D. High sensitive tellurium based {NO2} gas sensor. Sens. Actuators, B 2001, 73, 35−39. (29) Tsiulyanu, D.; Marian, S.; Liess, H.-D.; Eisele, I. Effect of annealing and temperature on the {NO2} sensing properties of tellurium based films. Sens. Actuators, B 2004, 100, 380−386. (30) Zhang, J.; Liu, X.; Neri, G.; Pinna, N. Nanostructured Materials for Room-Temperature Gas Sensors. Adv. Mater. 2016, 28, 795−831. (31) Liang, H.-W.; Liu, S.; Yu, S.-H. Controlled Synthesis of OneDimensional Inorganic Nanostructures Using Pre-Existing OneDimensional Nanostructures as Templates. Adv. Mater. 2010, 22, 3925−3937. (32) Liu, J.-W.; Huang, W.-R.; Gong, M.; Zhang, M.; Wang, J.-L.; Zheng, J.; Yu, S.-H. Ultrathin Hetero-Nanowire-Based Flexible Electronics with Tunable Conductivity. Adv. Mater. 2013, 25, 5910− 5915. (33) Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (34) Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758−1775. (35) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple [Phys. Rev. Lett. 77, 3865 (1996)]. Phys. Rev. Lett. 1997, 78, 1396−1396. (36) Liu, J.-W.; Chen, F.; Zhang, M.; Qi, H.; Zhang, C.-L.; Yu, S.-H. Rapid Microwave-Assisted Synthesis of Uniform Ultralong Te Nanowires, Optical Property, and Chemical Stability. Langmuir 2010, 26, 11372−11377. (37) Liu, J.-W.; Xu, J.; Liang, H.-W.; Wang, K.; Yu, S.-H. Macroscale Ordered Ultrathin Telluride Nanowire Films, and Tellurium/Telluride Hetero-Nanowire Films. Angew. Chem., Int. Ed. 2012, 51, 7420−7425. (38) Wang, K.; Yang, Y.; Liang, H.-W.; Liu, J.-W.; Yu, S.-H. First subkilogram-scale synthesis of high quality ultrathin tellurium nanowiresz. Mater. Horiz. 2014, 1, 338−343. (39) Liang, H.-W.; Liu, S.; Gong, J.-Y.; Wang, S.-B.; Wang, L.; Yu, S.H. Ultrathin Te Nanowires: An Excellent Platform for Controlled Synthesis of Ultrathin Platinum and Palladium Nanowires/Nanotubes with Very High Aspect Ratio. Adv. Mater. 2009, 21, 1850−1854.
for providing the computational facilities. S.T., A.R., and N.R.S. acknowledge the support from Advanced Facilities for Microscopy and Microanalysis (AFMM), IISc, for carrying out the electron microscopy characterization.
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