J. Phys. Chem. C 2009, 113, 16337–16341
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Electrodeposition and Characterization of Quasi-One-Dimensional Lead/Lead Selenide Heterostructure Ordered Arrays Shuangming Wang, Chen Wang, Mingzhe Zhang,* Zhaocun Zong, Huifang Tian, Chang Liu, Jing Cao, and Guangtian Zou State Key Laboratory of Superhard Materials, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed: March 17, 2009; ReVised Manuscript ReceiVed: July 10, 2009
Pearl-necklace-like Pb/PbSe heterostructure ordered arrays have been fabricated via a simple electrodeposition method. The phase structures, morphologies, and chemical compositions of the samples have been characterized by atomic force microscopy, field emission scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Near-infrared absorption spectra of as-prepared Pb/PbSe heterostructure ordered arrays exhibit a blue shift. Raman spectra have been obtained using a confocal Raman spectrometer. The I-V curves show oscillations and nonlinear, symmetric characteristics under different potentials at 15 K. Introduction One of the important goals of nanomaterials research is the development of experimental methods to selectively control the composition and shape of nanomaterials.1-3 Recently, both composition and shape control have been applied to semiconductor nanomaterials and metal-semiconductor nanostructures,4,5 which result in discrete domains of different materials joined together to form disparate material systems (such as metal/ semiconductor,1,3,6 magnet/semiconductor,6 or metal/magnet6,7 systems). Metal-semiconductor nanostructures are important materials whose characteristics lead to the development of heterojunction sturctures8 and devices, such as resonant tunneling diodes (RTDs). As an important narrow band gap groups IV-VI semiconductor, PbSe is an attractive material characterized by a low effective mass of both electrons and holes and an exceptionally large exciton Bohr radius of about 46 nm,9 which results in a strong confinement effect and large optical nonlinearity.10,11 Quantum-confined PbSe nanocrystals have been exploited for transistors,12 multiexciton generation,13,14 polymer-composite infrared photodetectors,15 and electroluminescence devices.16 Over the past few years, various methods have been reported for synthesizing PbSe nanostructures with different shapes, such as nanorods,17 nanotubes,18 nanowires,19-22 nanorings,20 nanocubes,23 and other nanocrystals.24 Among these methods, electrodeposition is one of the important methods. Although Pb and PbSe nano- and microstructures have been prepared by electrodeposition,25-27 pearl-necklace-like Pb/PbSe metal-semiconductor heterostructures have not been reported. Electrodeposition is capable of producing many regular nanoand microstructures28-31 that cannot be prepared with other traditional techniques. Different ions possess different deposition electrode potentials, so different components are deposited by varying amplitudes of deposition potential. The scale of each deposited component can be controlled by altering the width of the waveform for the deposition potential.32,33 In this work, according to the solute partition principle, we constructed a quasi-two-dimensional growth space between a * To whom correspondence should be addressed. Tel: +86 431 85166089. Fax: +86 431 85166089. E-mail:
[email protected].
silicon substrate and a cover glass to prepare Pb/PbSe pearlnecklace-like quasi-one-dimensional heterostructure ordered arrays with rodlike areas of 1.0-3.0 µm in length. Moreover, we investigated the electrical properties of the samples. Experimental Section Chemicals. The reagents, including lead nitrate, selenium dioxide, and nitric acid, were analytically pure and were used as purchased without further purification. Distilled water (resistivity, 18.2 MΩ · cm) was used in the experiments. Preparing Solution. In a typical procedure, 0.828 g of Pb(NO3)2 and 0.011 g of SeO2 were added to 50 mL of distilled water in a flask under stirring to form a white precipitate (PbSeO3). Subsequently, HNO3 was poured into the solution drop by drop to make the white precipitate just disappear. At this time, the pH of the solution was 1.550, as measured by a pH meter. All experimental processes were carried out at room temperature. Experimental Device. The growth system consisted of a growth chamber, a low-temperature cycle water bath, a dc power supply (DF1731SB5A), an arbitrary function generator (AFG 310), and a CCD camera (A311f). The growth chamber consisted of a piece of silicon substrate (20 × 20 mm2) with a SiO2 surface, on top of which two electrodes (cathode and anode) were placed and covered with a clean cover glass. The two parallel electrodes were made of a 50-µm-thick lead foil and separated by a distance of 8 mm. A peltier element under silicon substrate was fixed in the center of the growth chamber and was used to rapidly change the electrolyte temperature. The growth chamber temperature was controlled by the lowtemperature cycle water bath. Experiment Process. First, a silicon wafer was placed on the peltier element. Two parallel electrodes were put on the silicon substrate, and 0.05 mL of the prepared solution was dropped on the silicon substrate. After that, a cover glass was put on the two electrodes carefully to make the space between the cover glass and the silicon substrate full of solution. An ultrathin ice layer could be formed between the silicon substrate and the cover glass by adjusting the temperature of the peltier element and the low-temperature cycle water bath accurately. After the ultrathin ice layer was formed, we kept the growth
10.1021/jp902372h CCC: $40.75 2009 American Chemical Society Published on Web 08/21/2009
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Figure 1. (a) Applied voltage with periodic variation. (b) Each wire has a periodic pearl-necklace-like structure, where the path of the atomic force microscope measurement and the measured profile are also shown. (c) Enlarged view of a wire showing the polycrystalline appearance.
chamber temperature constant for 1 h. At this time, due to solute segregation, an ultrathin layer of concentrated PbSe electrolyte was formed between the ice layer and the lower silicon substrate as well as the ice layer and the higher cover glass separately. After freezing for 1 h, experiments were carried out by imposing a 1 Hz square wave with an amplitude of 0.2 V and an offset of 0.3 V across the parallel electrodes. The growth time was about 0.5 h, during which time real-time observation of the growth process could be made by an optical microscope (Leica Dmlm). When the growth process finished, the cover glass and silicon wafer were taken out, cleaned, and dried at room temperature. Preparation of the Sample for TEM. The samples could remain between the silicon substrate and the cover glass successfully by adjusting the growth chamber temperature. After the growth process ended, the cover glass was taken away and a TEM grid was put on the silicon substrate. Subsequently, the silicon substrate was heated by the peltier element to make the ice layer on the silicon substrate just melt. By doing this, the sample would be left on the TEM grid. Characterization. Transmission electron microscopy (TEM) characterization was performed on a JEM-2100F operating at 200 keV. A JEOL JSM-6700F field emission scanning electron microscope (FESEM) was used to image the nanowires. An energy-dispersive X-ray spectroscope (EDS) equipped in the SEM system was used to determine the sample compositions. AFM images were recorded on an atomic force microscope (model Nanoscope IIIa). Raman spectra were obtained by a Raman spectrometer (model Renishaw Invia). Fourier transform infrared (FTIR) data were taken on a MPA NIR spectrometer. The electrical properties were measured by a system consisting of digital phosphor oscilloscopes (Tektronix, TDS5034B), a digital source meter (Keithley, model 2400), low-temperature devices (American CCs-150), a molecular pump, and a digital display compound vacuum gauge. Results and Discussion Morphology and Composition Analysis. In the electrodeposition process, a periodic varying square wave (AFG310 supply) was applied across the electrodes, as shown in Figure 1a. The square wave was programmed using the arbitrary function generator (AFG310) and the digital phosphor oscilloscope (TDS5034B). When the frequency of the square wave is 1 Hz and the voltage varies from 0.4 to 0.8 V, we can see the formation of ordered arrays, as shown in Figure 1 (optical
Figure 2. (a) SEM image of two parallel heterostructure wires. (b) TEM image showing the pearl-necklace-like structure of the wires. (c, d) The electron diffraction patterns corresponding to marked areas in (b). HRTEM images shown in (e, f) correspond to (c, d), respectively.
microscope Leica Dmlm was employed), where each wire has a periodic pearl-necklace-like structure, that is, rodlike and knotlike areas corresponding to the high and low potential of the applied voltage, respectively. Figure 1b shows an AFM picture of the partial sample. The path of the atomic force microscopy measurement and the profile of such measurement showing the height of the wire, of the order of 230 nm, are delineated in the adjacent figure. From the SEM image shown in Figure 1c, the wires grown on the substrate have a polycrystalline appearance. Figure 2a shows an SEM image of two parallel wires. It can be seen that the deposit morphology is pearl-necklace-like. The rodlike areas are about 1.5 µm long. Figure 2b shows the TEM image of the sample. It can be seen that each wire has the knotlike and rodlike areas, which correspond to Pb and PbSe, respectively. This is confirmed by electron diffraction patterns, as shown in Figure 2c,d. Figure 2e,f shows the HRTEM images of the Pb and PbSe, corresponding to Figure 2c,d, respectively. The spacing of 0.174 nm corresponds to the (220) planes of Pb in Figure 2e, and the spacing of 0.216 nm corresponds to the (220) planes of PbSe in Figure 2f. The representative EDS patterns of the knotlike and rodlike areas of samples on the cover glass are shown in Figure 3a,b, respectively. Figure 3a shows only the presence of Pb elements except for the main composition of the glass (O, Si, Na, Ca), which indicates a high purity of Pb in the knotlike areas. Se and Pb elements are found in Figure 3b, which demonstrates the presence of PbSe in the rodlike areas. Growth Mechanism. As we know, different ions have different reduction potentials. When a periodic potential is applied across the two electrodes with varying amplitude, different ions can be deoxidized selectively to control the deposition of different components and form periodic heterostructure arrays. The composition and the growth velocity of deposition are closely related to the electrolyte and potential. In the experiments, a voltage of the square wave was applied
Quasi-1D Pb/PbSe Heterostructure Ordered Arrays
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Figure 3. EDS spectrum of (a) the knotlike areas and (b) the rodlike areas.
Figure 4. Room-temperature Raman spectrum for quasi-one-dimensional Pb/PbSe heterostructure ordered arrays.
Figure 5. Room-temperature absorption spectrum for quasi-onedimensional Pb/PbSe heterostructure ordered arrays.
across the electrodes. The ions would move toward the cathode in response to the varying electric field, leading to a varying concentration of ions near the cathode. In the growing process of each periodic structure, the bottom of the applied square waveform voltage corresponds to the growth of Pb nanograins, whose electrode reaction can be written as Pb2+ + 2e- f Pb (underpotential deposition ) UPD). The upper part of the applied square waveform voltage corresponds to the growth of PbSe nanograins, whose electrode reaction can be written as H2SeO3 + Pb2+ + 6e- + 4H+ f PbSe + 3H2O (overpotential deposition ) OPD).34,35 As the applied square waveform voltage reaches the bottom, the drift velocity of the migrating ions decreases, and the rate of deoxidization is reduced. This causes only part of the migrated Pb ions to be deoxidized, leaving the surplus Pb ions staying at the front of the electrodeposit. This high ion concentration results in an overall sluggish growth of the pattern, leading to more nucleations and transverse growth, constituting the knotty area of the wires (Pb). When the applied voltage increases to the upper part of the square wave, the drift velocity of the migrating ions increases, and a large number of nuclei quickly pile up. This results in a fast linear growth, constituting the rodlike area of the wires(PbSe). Optical Measurements. Figure 4 shows the room-temperature Raman spectrum of the Pb/PbSe sample. Raman peaks at 137 and 274 cm-1 come from PbSe. As shown in the spectrum, the dominant peak is at 137 cm-1. The longitudinal optical (LO) phonon is designated to the dominant peak at 137 cm-1.36 The weaker peak at 274 cm-1 is associated with two-phonon scattering (2LO).
The LO phonon should be at 135 cm-1, according to a theoretical calculation at 4.2 K. Comparing the measured Raman results to the calculations, we can see that the Raman shifts measured at room temperature are larger than that predicted by the calculation. The blue shifts may be attributed to the temperature effects. The Raman shifts, owing to temperature effects, can be qualitatively understood in terms of both of the anharmonic forces coupled to other vibrations at the Raman phonon and thermal expansion of the lattice in the crystal.37 Nanoscale PbSe has small, nearly equal carrier effective masses and a bulk exciton Bohr radius of 46 nm such that carriers experience strong confinement effects.38,39 Such strong confinement can potentially affect the balance between bulk and nanoscale influences. A room-temperature NIR absorption spectrum for Pb/PbSe nanoheterostructures is shown in Figure 5. We can see that the most prominent absorption peak is 5372 cm-1, which was observed in most samples and came from the PbSe area of the nanoheterostructures. As expected from quantum confinement, the onset of absorption is shifted substantially to the blue of the bulk band gap of 0.28 eV, with the wires having a shift of 0.39 eV.40 Electrical Properties. The quasi-one-dimensional Pb/PbSe heterostructure wires were further characterized by an electrical measurement system. Utilizing the nonconductive nature of ordinary cover glass, we can select a single wire on a glass slide for our measurement. When molybdenum films were used as the electrode deposited on a glass slide by magnetron sputtering, a single wire was linked to the circuit. Figure 6a shows the I-V characteristic curve of a single Pb/PbSe nanowire at 15 K. The voltage range used was from -0.005 to 0.005 V.
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Wang et al. knotlike appearance corresponding to the growth of Pb nanograins. Raman peaks at 137 and 274 cm-1 come from PbSe. The blue shifts of the Raman spectrum may be attributed to the temperature effects. The NIR absorption spectrum exhibits a blue shift due to quantum confinement. The I-V characteristics show a prominent feature of oscillations. Understanding these growth processes and the differences between similar material systems will assist in the rational design of other metalsemiconductor heterostructures with nano- and microstructures and their applications in future technologies. Acknowledgment. This work was funded by the Ministry of Science and Technology of China (No. 2005CB724404). It was also supported by the National Science Foundation of China (Nos. 50672029 and 20873052). References and Notes
Figure 6. (a) I-V characteristic curve of a single quasi-onedimensional Pb/PbSe heterostructure wire. The voltage range was from -0.005 to 0.005 V. (b) The I-V characteristic curve shows a nonlinear, symmetric I-V characteristic, in agreement with tunnel conductance. The voltage range was from -3 to 3 V.
As shown in Figure 6a, conductance shows oscillations. This result must occur while electrons move perpendicularly toward the double potential barrier. Tunneling theory is proposed to explain the origin of oscillating phenomena. Metal-semiconductor contact can form a potential barrier, so the sample forms double potential barriers. When a constant voltage is applied across the sample, the energy band would tilt, and the Fermi level of electrons from the emitter would reach or exceed the electron ground-state energy level. Therefore, electrons have a relatively large probability of crossing the double potential barrier by resonant tunneling. When the electron ground-state energy level is lower than the bottom of the conduction band under a certain voltage, resonant tunneling will be cut off, leading to negative differential resistance. If the electron is resonant with excited levels, the I-V curve can have an oscillatory appearance. As shown in Figure 6b, the I-V curve exhibits a nonlinear, symmetric variation, in agreement with tunneling conductance,41,42 which means that a periodically built-in potential barrier is poorly formed in the sample structure. The potential barriers formed at the interfaces reduce the flow of charge carriers across the interface.43 As mentioned previously, there are two possible carrier transport mechanisms that may occur between the metal-semiconductor junction and the intercrystallite boundaries, namely, thermionic emission and tunneling. However, at low temperature (15 K), thermionic emission was suppressed.43,44 Conclusions We have demonstrated the growth of Pb/PbSe quasi-onedimensional ordered arrays. Electrodeposition can construct a pattern of ordered lines, where every wire has a periodic pearlnecklace-like structure corresponding to the applied voltage. The higher voltage results in a rodlike area corresponding to the growth of PbSe nanograins, and the lower voltage results in a
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