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Register a new userThin film synthesis of semiconductors in the Mg-Sb-N materials system
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Nitrides feature many interesting properties, such as a wide range of bandgaps suitable for optoelectronic devices including light-emitting diodes (LEDs), and piezoelectric response used in microelectromechanical systems (MEMS). Nitrides are also significantly underexplored compared to oxides and other chemistries, with many being thermochemically metastable, sparking interest from a basic science point of view. This paper reports on experimental and computational exploration of the Mg-Sb-N material system, featuring both metastable materials and interesting semiconducting properties. Using sputter deposition, we discovered a new Mg2SbN3 nitride with a wurtzite-derived crystal structure and synthesized the antimonide-nitride Mg3SbN with an antiperovskite crystal structure for the first time in thin film form. Theoretical calculations indicate that Mg2SbN3 is metastable and has properties relevant to LEDs and MEMS, whereas Mg3SbN has a large dielectric constant (28epsilon_0) and low hole effective masses (0.9m_0), of interest for photovoltaic solar cell absorbers. The experimental solar-matched 1.3 eV optical absorption onset of the Mg3SbN antiperovskite agrees with the theoretical prediction (1.3 eV direct, 1.1 eV indirect), and with the measurements of room-temperature near-bandgap photoluminescence. These results make an important contribution towards understanding semiconductor properties and chemical trends in the Mg-Sb-N materials system, paving the way to future practical applications of these novel materials.
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We report a scalable approach to synthesize a large-area (up to 4 mm) thin black phosphorus (BP) film on a flexible substrate. We first deposited a red phosphorus (RP) thin-film on a flexible polyester substrate, followed by its conversion to BP in a high-pressure multi-anvil cell at room temperature. Raman spectroscopy and transmission electron microscopy measurements confirmed the formation of a nano-crystalline BP thin-film with a thickness of around 40 nm. Optical characterization indicates a bandgap of around 0.28 eV in the converted BP, similar to the bandgap measured in exfoliated thin-films. Thin-film BP transistors exhibit a field-effect mobility of around 0.5 cm2/Vs, which can probably be further enhanced by the optimization of the conversion process at elevated temperatures. Our work opens the avenue for the future demonstration of large-scale, high quality thin-film black phosphorus.
The recent discovery of high thermoelectric performance in Mg$_3$Sb$_2$ has been critically enabled by the success in $n$-type doping of this material, which is achieved under Mg-rich growth conditions, typically with chalcogens (Se, Te) as extrinsic dopants. Using first-principles defect calculations, we previously predicted that higher electron concentrations ($sim10^{20}$ cm$^{-3}$) can be achieved in Mg$_3$Sb$_2$ by doping with La instead of Se or Te. Subsequent experiments showed that free electron concentration in La-doped Mg$_3$Sb$_{2-x}$Bi$_x$ indeed exceeds those in the Te-doped material. Herein, we further investigate $n$-type doping of Mg$_3$Sb$_2$ and predict that, in addition to La, other group-3 elements (Sc, Y) are also effective as $n$-type dopants; Y is as good as La while Sc slightly less. Overall, we find that doping with any group-3 elements should lead to higher free electron concentrations than doping with chalcogens.
The recent discovery of n-type Mg$_3$Sb$_2$ thermoelectric has ignited intensive research activities on searching for potential n-type dopants for this material. Using first-principles defect calculations, here we conduct a systematic computational screening of potential efficient n-type lanthanide dopants for Mg$_3$Sb$_2$. In addition to La, Ce, Pr, and Tm, we find that high electron concentration ($geq$ 10$^{20}$ cm$^{-3}$ at the growth temperature of 900 K) can be achieved by doping on the Mg sites with Nd, Gd, Ho, and Lu, which are generally more efficient than other lanthanide dopants and the anion-site dopant Te. Experimentally, we confirm Nd and Tm as effective n-type dopants for Mg$_3$Sb$_2$ since doping with Nd and Tm shows superior thermoelectric figure of merit zT $geq$ 1.3 with higher electron concentration than doping with Te. Through codoping with Nd (Tm) and Te, simultaneous power factor improvement and thermal conductivity reduction are achieved. As a result, we obtain high zT values of about 1.65 and 1.75 at 775 K in n-type Mg$_{3.5}$Nd$_{0.04}$Sb$_{1.97}$Te$_{0.03}$ and Mg$_{3.5}$Tm$_{0.03}$Sb$_{1.97}$Te$_{0.03}$, respectively, which are among the highest values for n-type Mg$_3$Sb$_2$ without alloying with Mg$_3$Bi$_2$. This work sheds light on exploring promising n-type dopants for the design of Mg$_3$Sb$_2$ thermoelectrics.
Group-10 transition metal dichalcogenides (TMDs) are rising in prominence within the highly innovative field of 2D materials. While PtS2 has been investigated for potential electronic applications, due to its high charge-carrier mobility and strong layer-dependent bandgap, it has proven to be one of the more difficult TMDs to synthesise. In contrast to most TMDs, Pt has a significantly more stable monosulfide, the non-layered PtS. The existence of two stable platinum sulfides, sometimes within the same sample, has resulted in much confusion between the materials in the literature. Neither of these Pt sulfides have been thoroughly characterised as-of-yet. Here we utilise time-efficient, scalable methods to synthesise high-quality thin films of both Pt sulfides on a variety of substrates. The competing nature of the sulfides and limited thermal stability of these materials is demonstrated. We report peak-fitted X-ray photoelectron spectra, and Raman spectra using a variety of laser wavelengths, for both materials. This systematic characterisation provides a guide to differentiate between the sulfides using relatively simple methods which is essential to enable future work on these interesting materials.
Bi2Te3-xSex family has been the n-type start-of-the-art thermoelectric materials near room temperatures (RT) for more than half-century, which dominates the active cooling and novel waves harvesting application near RT. However, the drawbacks of brittle nature and Te-containing restrict the further applications exploring. Here, we show that a Mg3+{delta}SbxBi2-x family ((ZT)avg =1.05) could be a promising substitute for the Bi2Te3-xSex family ((ZT)avg =0.9-1.0) in the temperature range of 50-250 {deg}C based on the comparable thermoelectric performance through a synergistic effect from the tunable band gap using the alloy effect and the suppressible Mg-vacancy formation using interstitial Mn dopant. The former is to shift the optimal thermoelectric performance to near RT, and latter is helpful to partially decouple the electrical transport and thermal transport in order to get an optimal RT power factor. A positive temperature-dependence of band gap suggested this family is also a superior medium-temperature thermoelectric material for the significantly suppressed bipolar effect. Furthermore, a two times higher mechanical toughness, compared with Bi2Te3-xSex family, consolidates the promising substitute for the start-of-art n-type thermoelectric materials near RT.
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