ARTICLE pubs.acs.org/JPCC
Electrodeposited Single Crystalline PbTe Nanowires and Their Transport Properties Hyunsung Jung,† Deok-Yong Park,‡ Feng Xiao,† Kyu Hwan Lee,§ Yong-Ho Choa,|| Bongyoung Yoo,*,^ and Nosang V. Myung*,† †
)
Department of Chemical and Environmental Engineering and Center for Nanoscale Science and Engineering, University of California—Riverside, Riverside, California 92521, United States ‡ Department of Applied Materials Engineering, Hanbat National University, Daejeon, 305-719, Republic of Korea § Electrochemical Processing Group, Korea Institute of Materials Science, Changwon-Si, Kyungnam, 641-010, Republic of Korea Department of Chemical Engineering, Hanyang University, Ansan 426-791, Republic of Korea ^ Department of Materials Science and Engineering, Hanyang University, Ansan 426-791, Republic of Korea ABSTRACT: Single crystalline PbTe nanowires were potentiostatically electrodeposited by a template-directed method using track-etched polycarbonate membranes as scaffolds in acidic nitrate baths. They exhibited a face-centered cubic (FCC) structure with a preferred growth direction about 31 against the [200] direction. By galvanic displacing the ends of PbTe nanowire with gold prior to electrode microfabrication, the Schottky barrier (i.e., native PbTe oxide) at the interfaces between nanowire and electrodes was eliminated/reduced to form an ohmic contact between nanowire and electrodes. Field effect transistor (FET) transfer characteristics indicated that the electrodeposited single-crystalline PbTe nanowires are p-type semiconductors with the estimated field effect carrier mobility and concentration of 3.32 ( 0.15 cm2/(V s) and 1.85 ( 1.06 1018 cm-3, respectively.
’ INTRODUCTION Lead telluride (PbTe) is a narrow band gap (∼0.32 eV at 300 K) semiconductor with a remarkably large Bohr exciton radius (∼ 46 nm), excellent high quantum efficiencies, and a high thermoelectric figure-of-merit.1-4 In addition, their electrical and optical properties of PbTe strongly depend on the composition which allows altering peak wavelength and semiconducting characteristics.5 Because of these unique properties, PbTe has attracted intense scientific interest with potential applications in infrared (IR) detectors, laser diodes, and thermophotovoltaics and thermoelectric devices.6-9 One-dimensional nanostructures including nanowires and nanotubes have attracted a great deal of attention to further enhance the thermoelectric performance compared to bulk counterparts because of greater phonon scattering at the interfaces with a minor reduction of electrical conduction.10-12 A variety of physical and chemical techniques have been employed to synthesize PbTe nanowires in spite of the restriction of the cubic crystal structure favoring isotropic growth.13-15 For example, the solution-based chemical synthesis (e.g., two-step hydrothermal and one-step solvothermal polyol processes) and chemical vapor r 2011 American Chemical Society
deposition have been utilized to synthesize single-crystalline PbTe nanowires or nanocrystals. However, these methods typically lead to a broad size distribution in both diameter and length and require harsh operation conditions.16-20 Template-directed electrodeposition is an efficient method to synthesize nanowires because it can precisely adjust composition, crystallographic structure, texture, and grain size with controlled dimensions.5,21-24 Liu et al. demonstrated the ability to synthesize [111] preferred oriented PbTe single-crystalline nanowires using template-directed electrodeposition using an anodized alumina template as the scaffold.25 Yang et al. synthesized polycrystalline PbTe nanowires using the lithographically patterned nanowire electrodeposition (LPNE) method and demonstrated the ability to synthesize PbTe nanowires with controlled dimensions and composition.26,27 Our group also systematically investigated electrodeposition of PbTe thin films in acidic nitrate baths utilizing various electrochemical and material characterization techniques and demonstrated that crystal structures can be significantly altered Received: November 10, 2010 Revised: January 5, 2011 Published: February 2, 2011 2993
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by varying deposition potential and substrate.13,21 For example, single-crystalline PbTe cubes were electrodeposited on gold substrate by applying low overpotential.13 In this paper, single-crystalline PbTe nanowires with [200] growth direction were potentiostatically electrodeposited in acidic nitrate baths, and their morphology and crystallographic structure were investigated using SEM, TEM, and HRTEM. To determine electrical properties of a single nanowire, the galvanic displacement process was added to conventional electrode fabrication processes to eliminate/reduce contact resistance between electrodes and the nanowire. Using these single nanowire based devices, temperature-dependent electrical resistance (TCR) and field effect transistor (FET) transport properties were performed to determine their electrical properties.
’ EXPERIMENTAL PROCEDURE PbTe nanowires were potentiostatically electrodeposited using track-etched polycarbonate (PC) membranes (a normal pore diameter of 30 nm from Whatman Inc.) as scaffolds in acidic nitric baths. To form a working electrode, gold was sputtered on one side of a track-etched PC membrane to serve as a seed layer. The gold-sputtered PC membrane was attached to double-sided adhesive copper conducting tape, which was fixed on a glass slide. PbTe electrolytes were prepared by dissolving TeO 2 (99.9995%, Alfa Aesar, Inc.) and Pb(NO3)2 (99.7%, Fisher Chemical) in concentrated HNO3. Once oxide and salt were completely dissolved, deionized water was added to reach the final volume to make an electrolyte. The final concentration of HTeO2þ, Pb2þ, and HNO3 in the electrolyte was 0.001, 0.05, and 1 M, respectively. Ag/AgCl (in saturated KCl) was used as a reference electrode, and a platinum-coated titanium strip was used as a counter electrode. The electrodeposition was carried out at an applied potential of -0.12 V vs Ag/AgCl with magnetic stirring (1 in. long magnetic bar and 300 rpm) at room temperature. After finishing the electrodeposition of PbTe, the tracketched PC membrane was detached from copper tape and dissolved in 1-methyl-2-pyrrolidinone (99.5%, Alfa Aesar, Inc.) at 50 C for a few hours to completely dissolve the membrane. The suspended PbTe nanowires were centrifuged and then redispersed in isopropyl alcohol (IPA). The chemical composition of PbTe nanowires was determined by energy dispersive X-ray spectroscopy (EDS) (model IncaX-Sight, Oxford Instrument). Scanning electron microscopy (SEM) (model XLG-30FEG, Phillips) was used to observe surface morphology. The crystallography of nanowires was investigated using transmission electron microscopy (TEM) (model JEM-2100F, JEOL) operated at 300 kV acceleration. A high-resolution transmission electron microscopy (HRTEM) image and selected area electron diffraction (SAED) pattern also were obtained for microstructural analysis. Electrical properties of PbTe nanowire were measured using a source-measure unit (model 2601A, Keithley) with a coldfinger cryogenic system (model CCS-350SH, Janis) by varying the temperature from 300 to 10 K. Back-gated FET measurement was performed using a dual source measure unit (model 2636A, Keithley) at þ0.5 V drain-source voltage (VDS) varying gate bias from -25 to 30 V. ’ RESULTS AND DISCUSSION The cathodic electrodeposition of PbTe thin film in acidic medium has been reported in the previous work,21 which
Figure 1. SEM and TEM images showing electrodeposited PbTe nanowires at a potential of -0.12 V [vs Ag/AgCl (sat. KCl)]: (a) SEM, (b) bright-field TEM (with an inset of SAED pattern with zone axes of [0 1 2]), and (c) high-resolution TEM image.
consisted of two reaction steps. HTeO2 þ ðaqÞþ3Hþ ðaqÞþ4e-fTeðsÞþ2H2 O HTeO2 þ ðaqÞþPb2þ ðaqÞþ3Hþ ðaqÞþ6e-fPbTeðsÞþ2H2 O where PbTe was formed by the underpotential deposition (UPD) of Pb2þ onto the overpotential deposition of Te(s). Our prior work on the electrodeposition of PbTe thin films13 indicated that deposition potentials greatly influenced crystallographic structure and morphology with a narrow deposition potential (E = -0.12 V vs Ag/AgCl) which lead to the formation of single-crystalline cubes. On the basis of this work, the deposition potential was fixed at -0.12 V to form single-crystalline PbTe nanowires, even though the morphology, chemical composition, and dimension of PbTe nanowires can be feasibly tailored by the applied potential and concentration of electrolytes.13,21 The electrodeposited PbTe nanowires had narrow size distribution of diameter of 73 ( 8.6 nm and length of 3.6 ( 0.34 μm and displayed homogeneous morphologies. Figure 1 shows SEM and TEM images of electrodeposited PbTe nanowires. The PbTe nanowires exhibit a cigar-like shape which is predetermined by the geometry of pores in a track-etched PC template. An obvious spot pattern from selected area electron diffraction (SAED) indicates that these nanowires consisted of singlecrystalline PbTe with face-centered cubic (fcc) structure with a lattice parameter of a = 6.459 Å (inset at Figure 1b). By comparing the TEM image and the SAED pattern, it was determined that the growth direction of the nanowires has about a 31 angle against the [200] direction. It was also confirmed by HRTEM image of lattice fringes in Figure 1c that the PbTe nanowire exhibits a perfect order of crystalline lattice. The lattice spacing was measured to be approximately 0.321 nm which corresponded to the planes of fcc PbTe. The chemical composition of PbTe nanowires was measured to be approximately 42 atom % Pb which is somewhat off from the stoichiometric composition (50Pb50Te) with content variation of (3 atom % along the longitudinal direction. To improve device properties, it is critical to create an ohmic contact between nanowire and microfabricated electrodes.28 It has been investigated to improve electrical contact issues since device properties can be performed by material properties by itself. Except noble nanostructures including gold, platinum, and 2994
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Figure 2. Schematic of a new contact method to minimize/eliminate a contact resistance between PbTe nanowire and Au electrode for measuring the electrical properties of PbTe nanowire: (a) patterning of PR on a PbTe nanowire, (b) dipping PbTe nanowire assembly into an electrolyte for galvanic displacement, (c) formation of Au particles on ends of the PbTe nanowire, (d) formation of Au electrodes at the ends of a PbTe nanowire, and (e) PbTe nanowire before galvanic displacement and (f) after galvanic displacement.
Figure 3. Comparison of I-V characteristics of the PbTe nanowire and Au electrodes (a) before and (b) after galvanic displacement.
palladium, it is difficult to form an ohmic contact to reactive nanostructures because of rapid formation of native oxide. The native oxide may result in an increase of contact resistance between nanowire and electrodes during the measurement of electrical property. Even though a much more expensive procedure such as an e-beam lithography can include additional processes such as plasma etching to remove a native oxide layer or pattern four-point probes to measure the resistance without contact resistance, a new cost-effective method to improve the contact was developed to measure the “true” electrical property of a single nanowire with minimum contact resistance, which is schematically illustrated in Figure 2. The newly developed contact method efficiently removed the local area of the native oxide layer which minimized/eliminated contact resistance. The fabrication processes are as follows: First, the PbTe nanowires were dispensed on a highly doped p-type Si wafer with 300 nm thick SiO2 layer, followed by patterning of an electrode with exposing ends of the PbTe nanowire (Figure 2a). Prior to e-beam evaporation of electrode materials, the substrate was dipped into a gold plating solution including 20 mM Na3Au(SO3)2, ethylenediamine, and potassium fluoride for 6 h (Figure 2b). During this process, the exposed sections of PbTe nanowire are galvanically displaced by gold ions which lead to the formation of gold nanoparticles on the PbTe nanowire. Since galvanic displacement is an electrochemical process, gold nanoparticles are
directly formed on PbTe instead of native oxide (Figure 2c). Galvanic displacement (sometimes referred to as immersion plating or cementation) takes place because the reduction potential of a metal ion (i.e., gold ion) in solution is more positive than that of the sacrificial material (i.e., PbTe).29,30 After galvanic displacement of PbTe nanowire ends, electrodes were formed by e-beam evaporation of Cr (20 nm thick) and Au (180 nm thick). The electrodes were defined by lift-off techniques to form a single nanowire based device (Figure 2d). Figure 2e and f shows the surface morphologies of PbTe nanowire before and after galvanic displacement reaction, respectively, where formation of Au nanoparticles on the surface of PbTe nanowire was clearly observed (Figure 2f). Figure 3 shows the I-V characteristics of single PbTe nanowire with (a) and without (b) the galvanic displacement process. The I-V curves of the PbTe nanowire without galvanic displacement (Figure 3a) show a nonlinear behavior which indicates the presence of a Schottky barrier at the interface between the PbTe nanowire and gold electrodes. However, the PbTe nanowire with galvanic displacement shows a linear I-V curve (Figure 3b), which indicates that the Schottky barrier which may be caused by the native oxide layer on the PbTe nanowire was eliminated/reduced by Au displacement process, and ohmic contacts were established. Figure 4a shows temperature-dependent I-V characteristics of single PbTe nanowire after galvanic displacement treatment at 2995
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Figure 4. Electrical properties of single Pb42Te58 nanowire: (a) I-V characteristics as a function of temperature (with the inset of single Pb42Te58 nanowire based device), (b) electrical resistance as a function of temperature with calculated activation energies, (c) the schematic diagram of PbTe nanowire based device for back-gate FET measurement, and (d) a typical FET transport properties with a VDS of þ0.5 V.
the temperature range of 10-300 K. The inset in Figure 4a shows a SEM image of PbTe nanowire assembled across two Au electrodes. Although the I-V characteristic exhibited linear behavior at room temperature, the nonlinear I-V characteristic was displayed at low temperature. The Fermi level of p-type semiconductor at low temperature can be lower than the acceptor energy level, even though materials have an ohmic contact at room temperature. As the results, the linear behavior of I-V characteristics due to the ohmic contact can change to nonlinear behavior. As expected, the resistance of PbTe nanowire decreased with increasing temperature as shown in Figure 4b, which indicates the semiconducting behavior of electron transport in the PbTe nanowire. The thermal activation energy of PbTe nanowires, Ea, was estimated from the Arrhenius equation, which can be expressed with the following eq 1 Ea R ¼ R0 exp ð1Þ 2kT where R0 is the resistance at T = ¥; Ea is the thermal activation energy for electrical conduction; k is Boltzmann’s constant; and T is a temperature. From this relationship, Ea1 was calculated to be approximately 26 meV at the high-temperature range of 100-300 K. However, the slope was significantly changed and leveled off at the low-temperature range of below 40 K, and Ea2 was determined to be 0.2 meV in this temperature range. Compared to the intrinsic half energy band gap (Eg(300 K)/2 = 160 meV), the small activation energy of about 26 meV at high
temperature from 75 to 300 K may be due to some acceptor-like energy levels of Te-rich PbTe with nonstoichiometric defects. The abnormal small activation energy at low temperature from 10 to 50 K may be also attributed to excess Te (Pb vacancies) because highly doped PbTe can have acceptor levels with zero activation energy at 0 K.31,32 The transition behavior in the slope of resistance at the temperature range of 40-100 K strongly implies that the different electron transport mechanisms were existed at the high- and low-temperature regions. Similar transitions of the slope at high temperature (∼320 K) were reported in PbTe33,34 and CdSe34 thin films. Abd El-Ati34 suggested that the transition of conductivity (from n-type to p-type) with temperature in PbTe thin films is attributable to the increase in the number of migrating Pb vacancies. Also, it was reported that the predominant point defects are Pb vacancies or Te interstitial acceptors in PbTe thin films. It is well-known that a PbTe thin film always has an excess of Te, and the net hole concentration (p-n) can be altered by raising the temperature. Therefore, it is believed that the excess holes might result from the ionization of acceptor defects due to the presence of an excess amount of Te.34 According to Abd El-Ati, the increase of measuring temperature raises the number of ionized acceptor defects which migrate through the lattice to accumulate at the electrode surfaces. The transition point at high temperature (∼320 K) could not be observed in this study because our measurement was carried out in the temperature range of 10-300 K. However, such a transition of the slope at low temperature (∼220 K) in PbTe thin films was reported in other studies.33,36 They also reported 2996
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Table 1. Comparison of the Electrical Transport Properties of PbTe Nanowiresa resistivity (10-3 Ωm)
diameter (nm) synthesis method carrier type
a
field effect mobility (cm2/(V s)) carrier concentration (1018 cm-3)
ref
50-70
ED
p-type
0.27 ( 0.15
3.32 ( 0.15
1.85 ( 1.06
this work
20 84
ED
n-type
0.1-0.5 (no thermal treatment)
∼ 40
----
26,27
15.9-28.6 (thermal treatment)
3-7
10-30
solvo-thermal
p-type
2
----
----
18
83
CVD
p-type
1.2
0.71
84
19
60
CVT
n-type
∼7
0.83
0.88
37
30
hydrothermal
p-type
8.8
----
----
17
ED, electrodeposition; CVD, chemical vapor deposition; CVT, chemical vapor transport method.
that the activation energy and transition point were measured to be ∼0.106 eV and ∼200 K, respectively. It is obvious that the carrier concentration increased with rising temperature as typical semiconducting behavior from the temperature dependence of electron transport in the PbTe nanowires. Therefore, it is believed that the electrical conducting mechanism may be dominated by different impurity energy levels with increasing temperature. Using a few single nanowire based FET devices, the electrical transport properties of single nanowire were characterized in two-terminal configuration with the underlying Si substrate as the back-gate electrode on a 100 nm thick SiO2 dielectric layer as illustrated in the schematic diagram of Figure 4c. The gate voltage was swept from -25 to 35 V at fixed drain-source voltage of þ0.5 V. Figure 4d shows a typical IDS-VG curve with a fixed VDS of þ0.5 V. The channel current, IDS, continuously decreased with increasing gate potential, VG, which indicated that electrodeposited PbTe nanowires are a p-type semiconductor. Even though the charge transport mechanism from transistor characteristic analysis was not investigated in detail due to the limit of dielectric layer durability and instability of PbTe nanowire in air during the measurement, the field effect carrier concentration and mobility were determined using eqs 2 and 3 CVth Vth 2πεε0 p¼ ¼ ð2Þ eπr 2 L eπr 2 lnð2t=rÞ μ¼
2
dI L dVG CVDS
ð3Þ
where C is nanowire capacitance; Vth is threshold voltage; and r and L are the radius and length of nanowires, respectively. The capacitance of nanowire with respect to the back gate can be described with t being the thickness and the average dielectric constant of the dielectric layer (300 nm thick SiO2 with ε of 3.9). The mobility was determined by the measurement of transconductance (dI/dVG). On the basis of the IDS-VG curves, FET hole mobility and carrier concentration were determined to be 3.32 ( 0.15 cm2/(V s) and 1.8 ( 1.06 1018 cm-3, respectively. Compared to CVD- and CVT-PbTe nanowires, electrodeposited PbTe nanowires show greater field effect mobility. Table 1 compares the electrical properties of PbTe nanowires.17-19,26,27,37 Electrical measurement in the literature was carried out with PbTe nanowires and PbTe nanowire films excluding contact resistance which was produced by a four-point probe technique or postetching process.
’ CONCLUSION In this paper, single crystalline PbTe nanowires with a preferred growth direction about 31 against [200] direction were synthesized by electrodeposition in acidic nitrate baths. The galvanic displacement reaction process was introduced to deposit gold nanoparticles on PbTe nanowires to minimize/eliminate contact resistance between PbTe nanowire and electrodes. Electron-transport properties of these nanowires indicated that the electrodeposited PbTe nanowire is a low doped p-type semiconductor with excellent field effect mobility.
’ AUTHOR INFORMATION Corresponding Author
*Dr. B. Yoo. E-mail:
[email protected]. Tel.: 82-31-4005229. Fax: 82-31-417-3701. Dr. N.V. Myung. E-mail: myung@ engr.ucr.edu. Tel.: 951-827-7710. Fax: 951-827-5696.
’ ACKNOWLEDGMENT This research work was supported by the Pioneer Research Center Program through National Research Foundation of Korea (2010-0002231) funded by the Ministry of Education, Science and Technology (MEST) and the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. ’ REFERENCES (1) Urban, J. J.; Talapin, D. V.; Shevchenko, E. V.; Murray, C. B. J. Am. Chem. Soc. 2006, 128, 3248. (2) Murphy, J. E.; Beard, M. C.; Norman, A. G.; Ahrenkiel, S. P.; Johnson, J. C.; Yu, P.; Micic, O. I.; Ellingson, R. J.; Nozik, A. J. J. Am. Chem. Soc. 2006, 128, 3241. (3) Tritt, T. M. Science 1999, 283, 804. (4) Gelbstein, Y.; Dashevsky, Z.; Dariel, M. P. Phys. B: Condensed Matter 2005, 363, 196. (5) Xiao, F.; Hangarter, C.; Yoo, B.; Rheem, Y.; Lee, K.-H.; Myung, N. V. Electrochim. Acta 2008, 53, 8103. (6) Akimov, B. A.; et al. Semicond. Sci. Technol. 1993, 8, S447. (7) Feit, Z.; Kostyk, D.; Woods, R. J.; Mak, P. Appl. Phys. Lett. 1991, 58, 343. (8) Harman, T. C.; Taylor, P. J.; Walsh, M. P.; LaForge, B. E. Science 2002, 297, 2229. (9) Li, X.; Nandhakumar, I. S. Electrochem. Commun. 2008, 10, 363. (10) Hicks, L. D.; Dresselhaus, M. S. Phys. Rev. B 1993, 47, 16631. (11) Majumdar, A. Science 2004, 303, 777. (12) Natelson, D. Nat. Mater. 2006, 5, 853. (13) Xiao, F.; Yoo, B.; Bozhilov, K. N.; Lee, K. H.; Myung, N. V. J. Phys. Chem. C 2007, 111, 11397. (14) Zhang, B.; He, J.; Tritt, T. M. Appl. Phys. Lett. 2006, 88, 043119. 2997
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