Morphological evolution, growth mechanism, and magneto-transport properties of silver telluride one-dimensional nanostructures
© Li et al.; licensee Springer. 2013
Received: 22 June 2013
Accepted: 8 August 2013
Published: 20 August 2013
Single crystalline one-dimensional (1D) nanostructures of silver telluride (Ag2Te) with well-controlled shapes and sizes were synthesized via the hydrothermal reduction of sodium tellurite (Na2TeO3) in a mixed solution. The morphological evolution of various 1D nanostructures was mainly determined by properly controlling the nucleation and growth process of Ag2Te in different reaction times. Based on the transmission electron microscopy and scanning electron microscopy studies, the formation mechanism for these 1D nanostructures was rationally interpreted. In addition, the current–voltage (I-V) characteristics as a function of magnetic field of the highly single crystal Ag2Te nanowires were systematically measured. From the investigation of I-V characteristics, we have observed a rapid change of the current in low magnetic field, which can be used as the magnetic field sensor. The magneto-resistance behavior of the Ag2Te nanowires with monoclinic structure was also investigated. Comparing to the bulk and thin film materials, we found that there is generally a larger change in R (T) as the sample size is reduced, which indicates that the size of the sample has a certain impact on magneto-transport properties. Simultaneously, some possible reasons resulting in the observed large positive magneto-resistance behavior are discussed.
KeywordsSilver telluride One-dimensional nanostructures Morphological evolution Growth mechanism Magneto-transport properties
During the past few decades, a shape-controlled synthesis of semiconducting crystals with well-defined morphologies, such as belts, wires, rods, tubes, spheres, sheets, combs, and cubes, has attracted considerable attention due to their novel properties and applications in many fields [1–7]. Among these nanostructures, one-dimensional (1D) nanostructures have increasingly become the subject of intensive research due to their potential applications in a variety of novel devices [8–10]. The most prominent example is certainly the carbon nanotubes [11, 12]. Not only that, considerable efforts have been spent on the synthesis of nanobelts, nanowires (NWs), and other 1D nanostructures. Especially, with the miniaturization of devices in the future, searching for interconnects remains a challenge to future nanoelectronics. Therefore, it is essential to investigate 1D nanomaterials which can be applied in the nanoscale field.
As one typical example of the silver chalcogenides, Ag2Te has attracted increasing attention due to its much more technological prospects [10, 13, 14]. As reported, Ag2Te can transfer its structural phase from the low-temperature monoclinic structure (β-Ag2Te) to the high-temperature face-centered cubic structure (α-Ag2Te) at about 145°C [15, 16]. Low-temperature β-Ag2Te is a narrow band gap semiconductor with high electron mobility and low lattice thermal conductivity , which is desirable for its high figure of merit for thermoelectric applications. In the α-Ag2Te phase, silver cations can move freely, which enhance the conductivity, leading to superionic conductivity . More recently, it has been reported that Ag2Te is a new topological insulator with an anisotropic single Dirac cone due to a distorted antifluorite structure , leading to new applications in nanoelectronics and spintronics. It is also known that a huge large positive magneto-resistance (MR) has been observed in the case of silver telluride bulk samples  or thin films . However, to the best of our knowledge, the MR behavior of Ag2Te nanostructured materials is rarely reported. Here, we systematically investigate the current–voltage (I-V) characteristics under different magnetic fields and the extraordinary MR behavior of Ag2Te nanowires. The magneto-resistance can be strongly affected by the details of the Fermi surface geometry and character of electron–electron (e-e) interactions  and therefore gives valuable insight into the physics dominating the conductivity. Furthermore, Ag2Te with nontrivial MR can provide great opportunities in magnetic sensor and memory applications.
It was reported that Ag2Te tended to form 1D nanostructures. For instance, the rod-like structure of Ag2Te was synthesized by the method based on the template-engaged synthesis in which the Te nanorods were used as template reagents . Ag2Te nanotubes have been synthesized hydrothermally when sodium tellurite (Na2TeO3) and silver nitrate (AgNO3) in hydrazine/ammonia mixture were autoclaved at 393 K . Ag2Te NWs were obtained by cathodic electrolysis in dimethyl sulfoxide solutions containing AgNO3 and TeCl4 using porous anodic alumina membrane as the template . Recently, Ag2Te NWs were synthesized by a composite hydroxide-mediated method, where AgNO3 and Te powder were heated at 498 K in a Teflon vessel containing ethylenediamine and hydrazine hydrate . Samal and Pradeep  have developed a room-temperature solution-phase route for the preparation of 1D Ag2Te NWs. In addition, our research group has more recently reported the synthesis and electrical properties of individual Ag2Te NWs via a hydrothermal process . Herein, on this basis, we demonstrate a simple hydrothermal method for the synthesis of Ag2Te 1D nanostructures by employing ammonia acting as a complexing reagent and pH regulator hydrazine hydrate (N2H4 · H2O) acting as a reducing reagent. Very interestingly, we discovered the morphological evolution during the formation of 1D NWs. The morphological evolution for the 1D nanostructures is considered as the desired agent for understanding the growth mechanism and formation kinetics of crystals [26–28]. Therefore, we believe that this discoveryof the formation of 1D Ag2Te nanostructures could promote further studies and potential applications.
The materials used include Na2TeO3, AgNO3, aqueous hydrazine solution (80%) (N2H4 · H2O), and ammonia (25%) (NH3 · H2O). All of the reagents used in the experiment were directly used without further purification. The preparation of Ag2Te nanostructures involved a hydrothermal process as our previous works . In a typical experiment, 0.5 mmol of Na2TeO3 and 1.0 mmol of AgNO3 were dissolved in 15 mL of deionized water. After stirring for minutes, 0.40 mL of N2H4 · H2O (80%) and 0.40 mL of NH3 · H2O (25%) were dropped in the solution. A mixed solution was obtained and then transferred into a 25-mL Teflon-lined stainless steel autoclave, followed by heating at 160°C for a period of time in an electric oven. After heating, the autoclave was cooled down naturally to room temperature. After the hydrothermal treatment, the precipitate was collected and rinsed with distilled water and ethanol and then dried in air for further characterization. After a serious treatment, the as-synthesized sample was obtained for further characterization.
The size and morphology of the as-synthesized Ag2Te nanostructures were characterized using scanning electron microscopy (SEM) (JEOL JSM5600LV, Akishima-shi, Japan), equipped with X-ray energy dispersive analysis spectrum (EDS). The crystalline structure and chemical composition were characterized by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) and selected area electron diffraction (SAED) (JEOL 2010, operated at an accelerating voltage of 200 kV). X-ray photoelectric spectrum (XPS) (Kratos AXIS Ultra, Kratos Analytical, Ltd., Manchester, UK) and X-ray diffraction (XRD) (X’pert MRD-Philips, Holland). Thermogravimetric and scalable differential thermal analysis (TG-SDTA) was carried out at a heating rate of 10°C min−1 in N2 gas at a flowing rate of 50 mL min−1 using a TGA/SDTA851e system. The room-temperature Raman spectra of the Ag2Te NWs were recorded with a micro-Raman spectrometer (Renishaw 1000, Wotton-under-Edge, UK) equipped with a CCD detector and an Ar+ laser with a 514.5-nm excitation line (diameter of laser spot, 3 μm) and 4.2 mW of power. The MR of these device measurements were carried out at room temperature using a Quantum Design 9 T physical property measurement system (PPMS) with a rotational sample holder.
Results and discussion
In summary, a series of single crystalline 1D nanostructures of Ag2Te with well-controlled shapes and sizes were prepared by a facile one-pot hydrothermal synthesis approach. On the basis of these results, a rolling-up growth mechanism of the ultra-straight and long Ag2Te nanowires has been proposed. The formation of these 1D Ag2Te nanostructures can promote further studies and potential applications. Moreover, we systematically investigated the I-V characteristics and unusual MR behavior of the Ag2Te nanowires with monoclinic structure. It was found that the I-V of Ag2Te nanowires is more sensitive at low magnetic field, which reveals that the Ag2Te nanowires are suitable for low magnetic field sensor. In addition, the excellent single crystal quality with monoclinic structure raises the possibility for observing the unusual MR behavior in the as-prepared nanowires. Significantly, comparing to the bulk and thin film materials, we found that there is generally a larger change in R(T) as the sample size is reduced. This raises the possibility that the observed unusual MR behavior can be understood from its topological nature and may largely come from the surface or interface contributions.
X-ray energy dispersive analysis spectrum
Fast fourier transform
- N2H4 · H2O:
- NH3 · H2O:
Physical property measurement system
Selected area electron diffraction
Scanning electron microscopy
Transmission electron microscopy
Thermogravimetric and scalable differential thermal analysis
This work is financially supported by the National Natural Science Foundation of China (grant no. 20971036) and Changjiang Scholars and Innovative Research Team in University, no. PCS IRT1126, and the construct program of the key discipline in Hunan province (no.2011-76).
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