L. Giri‡
a,
G. Mallicka,
A. C. Jacksonb,
M. H. Griepa and
S. P. Karna*a
aUS Army Research Laboratory, WMRD, Aberdeen Proving Ground, MD-21005, USA
bAxalta Coating Systems, Wilmington, DE 19803, USA. E-mail: shashi.p.karna.civ@mail.mil
First published on 3rd March 2015
In order to synthesize defect free, highly crystalline single phase nanostructured bismuth chalcogenides, we have investigated the effects of several reaction conditions including, solvents, temperatures, reaction time, and reducing agents. A small variation in the reaction method resulted in Bi2Te3 with different morphologies, ranging from nanosize particles, rods, platelets, and tubes to nanosheets. The materials were characterized by powder X-ray crystallography, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray analysis, Raman spectroscopy, and four-probe current (I)–voltage (V) analysis. An optimized reaction condition allowed the synthesis of single-phase, impurity-free hexagonal nanoplates with size varying between 50 nm and 500 nm and thickness varying between 45 nm and 55 nm in a reproducible manner. The Raman spectra of the optimized hexagonal plates and sheets showed infra red (IR)-active modes around 118 cm−1 resulting from symmetry breaking, a characteristic feature of nanostructured Bi2Te3. Additional peaks at 94 cm−1 in the nanosheets, resulting from the surface phonon mode further confirmed the ultrathin Bi2Te3 structures. The I–V measurements on the optimized surface showed an n-type semiconducting behavior. The surface current measured as a function of applied voltage is two orders of magnitude higher than that across the stacked pellet in ambient conditions and much higher compared to previously published data on few quintuplet-thick Bi2Te3 nanofilms. The highlights of this study are the optimal solvothermic reaction conditions and their impact on obtaining defect free, highly crystalline single phase bismuth chalcogenides.
The synthesis of BixTey was carried out in two different media, namely (i) aqueous (A), and (ii) polyalcohol (P).
2NaBH4(aq) + 5Te + 8NaOH → 5Na2Te + 3H2(g) + 2B(OH)3(aq) + 2H2O | (1) |
3Na2Te + 2BiCl3(aq) → Bi2Te3 + 6NaCl | (2) |
The reaction solution turned to a black suspension after heating to reflux for approximately 90 minutes. 10 mL of this black solution was taken and the precipitate was separated by centrifugation and washed with water, ethanol, and acetone and dried in vacuum at 60 °C for 2 hours. The next sample was prepared by augmenting the above procedure by hydrothermal treatment and adjusting the pH. The pH of the final black solution (10) was increased to 12 by adding a few drops of 0.5 M NaOH. The remaining black solution was now put into a 50 mL Teflon-lined autoclave maintained at 120 °C for 12 h and then cooled to room temperature. The black precipitates were centrifuged and washed with distilled water, absolute ethanol and acetone in sequence. Finally, the dark products were dried in a vacuum at 60 °C for 2 h. Two additional samples were prepared by post synthesis treatment of the first sample with NaBH4 and reacting for 12 h at 120 °C and 180 °C, respectively. The pH value was adjusted to 12.4 before the hydrothermal treatment. The final products were collected by centrifuging the black suspensions washed with water, ethanol and acetone and dried in vacuum at 60 °C for 2 hours. Table 1 summarizes the reaction parameters utilized.
Sample | Solvent | Time (hours) | Temperature (°C) | Reducing agent | Mw of PVP |
---|---|---|---|---|---|
a The samples AM-3 and AM-4 are post synthesized with NaBH4. | |||||
AM-1 | H2O | 12 | 100 | NaBH4 | — |
AM-2 | H2O | 12 | 120 | NaBH4 | — |
AM-3 | H2O | 12 | 120 | NaBH4a | — |
AM-4 | H2O | 12 | 180 | NaBH4a | — |
PM-1 | EG | 12 | 180 | EG | 10![]() |
PM-2 | EG | 36 | 180 | EG | 10![]() |
PM-3 | EG | 12 | 180 | EG | 40![]() |
PM-4 | EG | 36 | 180 | EG | 40![]() |
To determine the electrical properties of bulk Bi2Te3, as prepared 50 mg of sample PM-4 was pressed under 1.5 GPa in a compressor to form a pellet.
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Fig. 1 SEM images of as prepared Bi2Te3 in aqueous medium (AM) showing flower-like (a and b), nano-tubular (c), and 2D nanosheets (d). XRD patterns of the samples are shown in (e). |
As observed from the images the morphology varies greatly with change in synthesis parameters from flower-like (Fig. 1a and b), nanotubular (Fig. 1c) to two-dimensional nanosheets (Fig. 1d). The samples obtained after the post synthesis treatment with NaBH4 show more refined nanostructures as in Fig. 1c and d. The EDS spectrum of these samples confirm the presence of Bi and Te (Fig. S2†). The chemical composition and the crystalline phase structure of the as prepared Bi2Te3 nanostructures are shown in Fig. 1e. Sample AM-1 has mainly peaks from BiOCl along with some characteristic lines of Bi2Te3 and TeCl4. The Bi2Te3 lines get stronger with the modification of the synthesis condition, such as changing the reaction temperature, carrying out the reaction in a hydrothermal vessel and post synthesis treatment with NaBH4. Sample AM-2 has prominent Bi2Te3 lines with fewer BiOCl. Sample AM-3 has mostly Bi2Te3 peaks with a couple of weak lines from BiOCl and TeCl4. Finally, for sample AM-4, all peaks in the diffraction pattern correspond to the reflections of rhombohedral Bi2Te3. No detectable impurity peaks were observed, indicating that it has the rhombohedral lattice structure of Bi2Te3 (space group of R3m).27 The XRD pattern of the sample AM-4 has the characteristic features corresponding to (006), (101), (015), (018), (1010), (110), (0015), (205), (208), (0210), (1115), (125), (2110) and (300) planes according to the standard JCPDS card no. 15-0863, The peaks labeled (015), (1010) and (110) are much stronger compared with other peaks in the X-ray pattern. The particle size of the sample AM-4 was estimated through the Debye–Scherrer equation, d = kλ/βcos
θ, where k is 0.93, λ is Cu Kα wavelength (1.54 nm), β is the full width half maximum of the intense peak and θ is Bragg angle. The average crystallite size found is ∼45 ± 10 nm.
The SEM images and corresponding XRD patterns of the samples prepared in polyol medium are shown in Fig. 2. The morphology of samples PM-1 (Fig. 2a) and PM-3 (Fig. 2c), which were obtained after 12 hours of reaction time with fixed temperature (180 °C) and separate molecular weights (10000 and 40
000) of reducing agents (EG), resembled regular rhombohedron, with particle size varying between 50 nm and 200 nm. The sample PM-3 had well separated particles of uniform shapes and was prepared using PVP K-30 with higher molecular weight. Samples PM-3 and PM-4 were obtained after 36 hours reaction time at 180 °C and 10
000 and 40
000 MW of EG, respectively. The morphology changed to stacked hexagonal shaped plates (Fig. 2b and d). The size of these nanoplates varied between 50 nm and 500 nm and the thickness varied between 50 nm and 60 nm. However, the hexagonal plates of PM-4 became thinner and separated (Fig. 2d).
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Fig. 2 SEM images of as prepared Bi2Te3 in polyol medium (PM) showing rhombohedron (a and c) and hexagonal platelet structures (b and d). XRD patterns of the samples are shown in (e). |
The XRD patterns of samples prepared in polyol medium are shown in Fig. 2e. Samples PM-1 and PM-3, which were obtained after 12 hours reaction time, shows some peaks from Bi along with the Bi2Te3. Samples PM-2 and PM-4, obtained from 36 hours reaction time, show distinct peaks that can be indexed to the rhombohedral crystal system, (space group R3m), characteristic for Bi2Te3,27 confirming the final product to be single phase Bi2Te3. Sample PM-2 (PVP 10000 MW, 36 hours) has the most intense and the sharpest XRD peaks from the synthesized Bi2Te3 nanocrystals, which are typical signatures of a high degree of crystallinity as confirmed by HRTEM. Compared to Fig. 1e, some higher order reflections from (0111), (2011), and (0120) planes are also present as confirmed from the standard literature value.29 The calculated lattice constants, a = b = 4.385 Å and c = 30.487 Å are in very good agreement with the standard literature values (a = b = 4.385 Å and c = 30.483 Å; JCPDF #15-0863). The average estimated particle size for sample PM-2 is ∼40 ± 10 nm. The particles in samples PM-3 and PM-4 got smaller compared to those in PM-1 and PM-2 (Fig. 2e).
Since, samples AM-4 and PM-4 were mostly free of impurities we determined to investigate them further. The fine microstructure of the Bi2Te3 nanotubes and nanoparticles were also studied by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM).
The TEM images of sample AM-4 (Fig. 3a) (also see Fig. S1 in the ESI†) show irregularly shaped nanoparticles including nanorods. The HRTEM image of AM-4 in Fig. 3a show structurally uniform lattice fringes, with a spacing of 0.22 nm and 0.32 nm, in good agreement with the d value of the (110) and (015) planes, respectively, of rhombohedral Bi2Te3 according to the standard JCPDS card no. 15-0863.
Sample PM-4, prepared in the polyol medium, consisted of uniformly distributed hexagonal nanoplates as seen from the TEM images (also see Fig. S3†). The size of these nanoplates, measured between the opposite sides of the hexagons, varied from 50 nm to 500 nm. The thickness of a single nanoplate, measured by electron energy loss spectroscopy (EELS), varied between 45 nm and 55 nm. The high contrast (dark over transparent crystals) areas in the TEM images (Fig. 3b and S3†) are evidence of the ultrathin nature of the nanocrystals.
The nanosheets exhibit a high degree of crystallinity, evidenced by flat surface and sharp edges. Ripple-like contrasts observed in some crystal surfaces most probably result from surface strain and/or non-uniformity in thickness, as also reported earlier.25
The HRTEM images were taken from the marked area of Fig. 3b. The lattice shown in the HRTEM image (upper inset of Fig. 3b) and the spot pattern of SAED (lower inset of Fig. 3b) demonstrate the crystalline nature of the nanoplate. The lattice fringes were structurally uniform with a spacing of 0.32 nm, which is in good agreement with the d-spacing of the (015) planes of rhombohedral Bi2Te3 (JCPDS card no. 15-0863). The SAED pattern taken from a single nanoplate confirms the hexagonal symmetry of the crystal plane. The bright diffraction spots are indicative of the high degree of crystallinity and also suggest the absence of planar structural defects. The d spacings calculated from the diffraction spots match with those obtained from PXRD pattern. The EDS spectrum of sample PM-4 (Fig. S2†) taken from the same marked area as in Fig. 3b show the presence of elemental bismuth and tellurium alone, confirming the purity of the final product. The Cu lines appearing in the spectrum are due to the copper grids of the sample holder.
The local atomic arrangements and the nature of chemical bonds in the synthesized nanosheets (sample AM-4) and nanoplates (sample PM-4) were also investigated by Raman spectroscopy (Fig. 3c and d). The Raman spectrum of bulk crystalline Bi2Te3 is known to exhibit three active modes, namely A11g at 61 cm−1, E2g at 102 cm−1, and A21g at 134 cm−1.30 The states labeled “E” and “A” indicated the in-plane and out-of-plane lattice vibrations, respectively. The subscript “g” (gerade for symmetric) denotes Raman-active, while “u” (ungerade for asymmetric) represents Raman-inactive but IR-active modes. In addition to the classical Raman-active modes, both samples show an additional peak around 118 cm−1 which could be ascribed to the A1u mode.26,31–36 Bi2Te3 is centro-symmetric and therefore A1u mode is Raman forbidden. However, for the nanostructured samples low dimensional structure breaks down the centro-symmetric nature of the bulk Bi2Te3 and A1u peaks appear in Raman spectrum.26,31–36 For the hexagonal nanoplates (sample PM-4), additional vibration modes also show up. The peak at 94 cm−1 results from the surface phonon mode (SPM), only observed in nano-sized materials and the peak S2 is an infrared-active mode due to the size effect.35 The origin of peaks labeled S1 and S3 could not be identified and needs further study to categorize them to defects in nanostructures.
To understand the electrical properties of the synthesized Bi2Te3 nanoplatelets, several measurements of current (I) with respect to drain voltage (V) were performed on the surface and across the carefully prepared pellet of Bi2Te3 at room temperature. Fig. 4 shows the I–V characteristics of the as prepared Bi2Te3 pellet probed by a semiconductor (SC) analyzer (Keithley-4200) attached to a four-probe micromanipulater system (Janis) at high (±3 V) (Fig. 4a) and low (±0.5 V) (Fig. 4b) source–drain voltage sweeps. The inset of Fig. 4b is the conductance (dI/dV) plot at low drain voltage. The I–V characteristics of the Bi2Te3 nanoplatelets surface show an n-type semiconducting behavior, with a distinct non-linearity for higher voltage sweep, qualitatively similar to that recently reported by Teweldebrhan et al.,36 but showing a rather much higher current, which shows a high purity, defect free single phase crystalline surface of the synthesized nanoplatelets. With the bottom of the pellet contacted to a metal (steel) gate, no gating effect was noticed even at a high gate potential of ±20 V, similar to that observed for a few quintuple-thick film samples previously reported.36 In the present case, this could result from a rather rough interface and loose packing between the nanostructured pellets. This is confirmed by a negligible current across the 0.5 mm pellet as shown in Fig. 4 for high and low voltage sweeps. High electrical current at the surface of the nanostructured pellets would be advantageous for electronic devices as well as for investigating quantum mechanical effects.
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Fig. 4 Current (I)–voltage (V) measurements of Bi2Te3 pellet surface (square) and bulk (circle) at high (a) and low (b) voltages. The conductance (inset) of the I–V is shown in (b). |
Footnotes |
† Electronic supplementary information (ESI) available: Additional TEM and EDS data on synthesized samples supplied. See DOI: 10.1039/c5ra02303c |
‡ Oakridge Institute of Science and Education (ORISE) Senior Associate. |
This journal is © The Royal Society of Chemistry 2015 |