Free-standing millimetre-long Bi2Te3 sub-micron belts catalyzed by TiO2 nanoparticles
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Physical vapour deposition (PVD) is used to grow millimetre-long Bi2Te3 sub-micron belts catalysed by TiO2 nanoparticles. The catalytic efficiency of TiO2 nanoparticles for the nanostructure growth is compared with the catalyst-free growth employing scanning electron microscopy. The catalyst-coated and catalyst-free substrates are arranged side-by-side, and overgrown at the same time, to assure identical growth conditions in the PVD furnace. It is found that the catalyst enhances the yield of the belts. Very long belts were achieved with a growth rate of 28 nm/min. A ∼1-mm-long belt with a rectangular cross section was obtained after 8 h of growth. The thickness and width were determined by atomic force microscopy, and their ratio is ∼1:10. The chemical composition was determined to be stoichiometric Bi2Te3 using energy-dispersive X-ray spectroscopy. Temperature-dependent conductivity measurements show a characteristic increase of the conductivity at low temperatures. The room temperature conductivity of 0.20 × 105 S m −1 indicates an excellent sample quality.
KeywordsNanowires Topological insulators Temperature-dependent conductivity Bismuth telluride
atomic force microscopy
energy-dispersive X-ray spectroscopy
focused ion beam
physical vapour deposition
scanning electron microscopy
topological surface state
Bi2Te3 is a well-known thermoelectric and a topological insulator (TI) . Interest in thermoelectrics is fuelled by the potential to generate power from waste heat [2, 3]. The thermoelectric efficiency is quantified by the figure of merit ZT which is a function of the electrical and thermal conductivity and the Seebeck coefficient of the thermoelectric material. Single-crystalline quasi-one dimensional structures on the nano- and sub-micron level are particularly suited to study surface effects such as morphological features or TI-based surface transport which is enhanced relatively due to the high surface-to-volume ratio [4, 5]. The topologically protected surface transport emerges as a result of strong spin-orbit coupling in Bi2Te3 and other materials . The surface state is formed by a single Dirac cone with linear dispersion and has attracted great interest in the last decade . It provides spin-momentum-locked electronic transport on the surface whilst the bulk of the material is a trivial insulator. In Bi2Te3, the bulk contribution to the total charge transport is very high which makes it challenging to characterize the topological surface state (TSS). It is one of the challenges in the field to overcome this hurdle by producing intrinsic materials with a high surface-to-volume ratio, such as single-crystalline nanowires, to effectively suppress the relative bulk contribution [8, 9].
The unit cell of Bi2Te3 consists of three quintuple layers (QLs) with the stacking sequence Te-Bi-Te-Bi-Te. Bi2Te3 nanowires grow parallel to these layers . Synthesis techniques include solvothermal growth , molecular beam epitaxy , on-film formation , and physical vapour deposition (PVD) , among others . However, the synthesized structures are often heterogeneous and short, as there are, e.g. platelets growing alongside wires which are less than some 10 μm in length. There is a profound interest in long Bi2Te3 nanowires for three reasons: (i) They enable the observation of pronounced Shubnikov-de Haas oscillations as seen in long Bi2Se3 nanowires ; (ii) they offer the possibility to combine multiple devices on a single nanowire ; and (iii) long nanowires are an interesting building block for sensors that require few high-aspect electrodes over a wide area without the need of high spatial resolution .
Previous work on vapour-liquid-solid-grown (VLS-grown) Bi2Te3 nanowires was limited to quasi-four-point-probe measurements due to the short length of the nanowires . The influence of contact resistance could not be fully excluded. Andzane et al. demonstrated four-point-probe measurements on Bi2Te3 nanobelts that were synthesized in a two-step process . Here, we report the one-step synthesis of free-standing millimetre-long Bi2Te3 sub-micron belts by PVD. The growth yield is increased by an unusual catalyst for nanostructure growth, namely TiO2, which was reported to outperform Au catalysts in the growth of Sb-doped Bi2Se3 nanowires . Four separated contacts are prepared by standard laser lithography to extract the temperature-dependent conductivity.
The PVD growth was carried out in a Nabertherm B180 horizontal tube furnace (Lilienthal, Germany) under constant nitrogen flow of 300 sccm at atmospheric pressure using Bi2Te3 powder as a precursor. The furnace was flushed with nitrogen several times after loading Si(100) substrates (downstream) and the Bi2Te3 precursor (upstream) into quartz boats. Then, the oven was ramped to the growth temperature of 600 °C and held constant for a growth time of typically 1 h. The samples were removed after the furnace cooled down to room temperature and subsequently analysed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and atomic force microscopy (AFM). Individual belts were placed on a silicon wafer with 300 nm of SiO2 field oxide by a mechanical transfer method that provides sub-micrometer precision . Four-point contacts were made by standard laser photolithography (using AZ3007 photoresist). After writing the pattern, the sample was H2-plasma cleaned at 100 W for 90 s. A 50 nm layer of Au was sputter-deposited and subsequently lifted off. The devices were glued into chip carriers using silver paint and wire bonded using Al wires. Temperature-dependent resistance curves R(T) were measured by current-voltage sweeps in four-point configuration (using a Keithley 6221 current source and 2182A nanovoltmeter). The samples were kept in a He atmosphere at ambient pressure. Subsequent to the transport measurements, cross sections were obtained using a Nova 600 NanoLab (FEI). First, the belt is covered with a platinum layer deposited at an electron voltage of 5 kV and a beam current of 0.4 nA. The platinum layer serves as protection layer and as thermal bridge to sink the heat during the focused ion beam etching. A gallium ion current of 50 pA is used at a voltage of 10 kV for etching.
Results and discussion
Summary of the electrical conductivity σ at room temperature, the belt width w B, height h B, and length of the central part l B, and the surface-to-volume-ratio S/V
(105 S m−1)
B1 has a conductivity of ∼0.5×105 S m−1, comparable to the low value reported for n-type bulk Bi2Te3 . The electrical conductivity of the two belts differs by nearly one order of magnitude. Stoichiometry variations beyond + 2 % Te in Bi2Te3, as determined by EDX on several belts, can be excluded as the cause of this difference. Further, deviations in electrical conductivity can originate from defects or Te-depletion near the surface that leads to a surface layer of high electrical conductivity . In our case, the surface-to-volume-ratio of B2 is about 20 % higher than that of B1 so that surface effects may be, at least in parts, the origin of the higher electrical conductivity of B2 .
The temperature-dependent electrical conductivity is shown in Fig. 5. Both belts show a minimum in the conductivity at an intermediate temperature of 185 K for B1 and 95 K for B2, respectively. A characteristic minimum in the conductivity has also been observed in Bi2Te3 bulk samples and nanostructures grown by different methods [23, 24, 25, 26, 27]. The feature appears at a temperature when the contribution of the surface conduction to the total conductivity becomes significant compared to the bulk contribution at a carrier density below 1×1017cm−1. Further studies will employ magnetoresistance measurements at low temperatures to distinguish between both contributions.
In summary, we have studied the growth of Bi2Te3 sub-micron belts using TiO2 nanoparticles as catalyst. The growth on substrates coated with the catalyst solution was compared to pristine Si substrates, overgrown under exactly the same conditions. The catalyst-coated substrates have a much higher belt yield; however, self-catalysed growth is also present. Very long belts can be grown from TiO2, with their length only limited by the growth time. For an 8-h growth, belts of up to ∼1 mm in length were produced. Their exceptional length makes these belts suitable candidates for electronic transport studies. The conductivity is as low as for pure bulk Bi2Te3 which is an advantage for the observation of the topological surface state and a sign of excellent crystal quality. Future work may explore the correlations between thermoelectric properties and the topological surface state further for an application of Bi2Te3 nanowires as versatile building blocks for thermoelectric, sensor, and spintronic devices.
We gratefully acknowledge Diamond Light Source for access to the Surfaces and Interfaces Laboratory. PS acknowledges partial funding by the EPSRC and Corpus Christi College (University of Oxford). This publication arises from the research funded by the John Fell Oxford University Press (OUP) Research Fund. FZ acknowledges the support by the Oxford University and USTC’s summer internship programme.
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