P-type Te Research Articles

Atomistic oxidation mechanism of Bi0.5Sb1.5Te3 (0001) surface

Surface oxidation can potentially influence transport properties of thermoelectric materials and service performance of thermoelectric devices. Here, the oxidation mechanism of the (0001) surface of a widely used p-type TE material, Bi0.5Sb1.5Te3 (BST), was investigated using spherical aberration corrected scanning transmission electron microscopy (STEM). Combining atomic resolution STEM and density function theory (DFT), it was revealed that oxidation occurs at room temperature in the dry air through O diffusion into the surface quintuple layer, facilitating formation of both cation and anion vacancies, weakening the Bi-Te and Sb-Te bonds, leading to oxidation product Sb2O3, amorphous Te, and Bi-rich Bi2Te3. Formation of a thin O-rich cation-deficient quintuple layer on the surface and the Bi-rich BST region can significantly reduce the interfacial reaction between BST and Ni electrode, as the reaction layer thickness is reduced by 75% after the surface oxidation. This work provides essential structural information on surface oxidation mechanism and its influence on properties and performance of TE materials and devices.

Photoluminescence properties of two-dimensional semiconductor heterointerfaces.

Two-dimensional metal-sulfur compounds have attracted much attention due to their novel physical properties, such as layered structure, ultrathin physical dimensions, and continuously tunable bandgap. The vertical stacking of different 2D semiconductors enables the heterojunction to retain the excellent properties of its constituent materials and has physical properties such as interlayer energy transfer and interlayer carrier transfer. In this paper, we utilize the carrier interlayer transfer properties of p-n heterojunctions and form heterojunctions using p-type Te and PdSe2 prepared with n-type monolayer WS2 using the microzone transfer technique. We found that the PL spectrum of monolayer WS2 is purer after heterojunction formation. The photoluminescence peaks representing exciton recombination are sharper, while the peaks represented by trions almost disappear. These phenomena indicate that we can utilize p-n junctions to capture the PL spectra of excitons in WS2, which is important for the further study of the optical properties of 2D metal-sulfur compounds.

Cyclic oxidation behaviour of N-type (Zr,Ti)Ni(Sn,Sb) and P-type (Zr,Ti)Co(Sn,Sb) thermoelectric materials

In this study, the fabricated Hf-free N-type (Zr,Ti)Ni(Sn,Sb) and P-type (Zr,Ti)Co(Sn,Sb) thermoelectric materials were subjected to cyclic oxidation testing at 500 °C for 10, 30, and 50 cycles. The oxidation behaviour of the materials was systematically investigated by evaluating mass gain to study the oxidation kinetics and by analysing surface morphology, phase constitution and elemental distribution to investigate the oxidation mechanism. The results indicated that both of the materials were severely oxidised during the cyclic oxidation testing, and the mass gain followed the parabolic kinetics and the parabolic rate constant (kp) being 0.006165 mg2cm−4s−1 and 0.000109 mg2cm−4s−1 for the N-type and the P-type TE materials, respectively. Alternated multilayers of Ni3Sn4+SnO2+(Zr,Ti)O2 and CoSb + SnO2+Sb2O4+(Zr,Ti)O2 were identified on the surface of the N-type and P-type materials, respectively, after the cyclic testing, which would deteriorate the thermoelectric performance of the materials. The outcome of this study strongly suggests that it is essential to improve the oxidation resistance and the thermal stability of the N-type (Zr,Ti)Ni(Sn,Sb) and P-type (Zr,Ti)Co(Sn,Sb) thermoelectric materials for high-temperature applications.

Construction of branched SnS2/Te heterostructure film for self-powered high-performance photodetector application

Effective design and construction of heterostructure can conduce to charges transportation/separation and simultaneously improve light absorption ability of photoelectrochemical (PEC) photoelectrode. In this work, the p-type Te nanorods were directly grown on the surface of ultra-thin SnS2 nanoflakes via a facile and low-cost electrochemical deposition (ED) method. The PEC performance of mixed-dimensional branched SnS2/Te nanoflakes/nanorods (NFRs) heterostructure composites film was optimized by varying the ED time. Under simulated light illumination of 40 mW/cm2, the optimal photocurrent density of the SnS2/Te NFRs photoanode significantly reached up to 0.95 mA/cm2, approximately 5.2 times and 3.5 times larger than those of bare SnS2 and Te film, respectively. The improved performance was attributed to appropriate energy band matching and intensive light absorption property. In particular, it presented good stability and high light on/off ratio of about 4.1 ×106 at the 0 bias voltages, suggesting an excellent self-powered characteristic of the device. Furthermore, the SnS2/Te NFRs photodetector exhibited photoresponsivity of 127 mA/W at 0.8 V bias and photodetectivity of 1.33 ×1011 Jones, respectively. This study presents a rational strategy to improve the photodetection performance of SnS2-based PEC-type photodetector and opens a new avenue for designing high-efficiency photoelectronics device.

Processes to enable hysteresis-free operation of ultrathin ALD Te p-channel field-effect transistors.

Recently, tellurium (Te) has been proposed as a promising p-type material; however, even the state-of-the-art results couldn't overcome the critical roadblocks for its practical applications, such as large I-V hysteresis and high off-state leakage current. We developed a novel Te atomic layer deposition (ALD) process combined with a TeOx seed layer and Al2O3 passivation to detour the limitations of p-type Te semiconducting materials. Also, we have identified the origins of high hysteresis and off current using the 77 K operation study and passivation process optimization. As a result, a p-type Te field-effect transistor exhibits less than 23 mV hysteresis and a high field-effect mobility of 33 cm2 V-1 s-1 after proper channel thickness modulation and passivation. Also, an ultralow off-current of approximately 1 × 10-14 A, high on/off ratios in the order of 108, and a steep slope subthreshold swing of 79 mV dec-1 could be achieved at 77 K. These enhancements strongly indicate that the previously reported high off-state current was originated from interfacial defects formed at the metal-Te contact interface. Although further studies concerning this interface are still necessary, the findings herein demonstrate that the major obstacles hindering the use of Te for ultrathin p-channel device applications can be eliminated by proper process optimization.

Ferrocene-modified polymer-SWCNT composite films for high-performance flexible thermoelectric generators

Highly efficient p-type TE composites based on poly(Fc-CPT)-based conjugated polymers are developed. Their morphological analysis and theoretical calculations elucidate the structure-property relationship in their thermoelectric performance.

Novel Thermoelectric Fabric Structure with Switched Thermal Gradient Direction toward Wearable In-Plane Thermoelectric Generators.

Wearable thermoelectric generators (TEGs) have exhibited great potential to convert the temperature gradient between the human body and the environment into electrical energy for maintenance-free wearable applications. A 2D planar device structure is widely employed for fabricating flexible TEGs due to its simple structure and facile fabrication properties. However, this device configuration is more appropriate for utilizing in-plane temperature differences than the out-of-plane direction, which limits their application in wearable cases since the temperature difference between the human body and the environment is in the out-of-plane direction. To solve this problem, a novel fabric-based TEG structure that can utilize the out-of-plane temperature gradient is proposed in this work. By introducing thermally conductive components in the generator, the out-of-plane temperature difference can be switched to the in-plane direction, which can be further utilized for 2D planar devices in wearable applications. The prepared thermoelectric fabric prototype with only 12 p-type TE legs exhibits a maximum open-circuit voltage of 4.69mV and an output power of 39.7 nW at a temperature difference of 30 K. This strategy exhibits a high degree of versatility and can be readily applied to other 2D planar TEGs, thus expanding their potential application in wearable technology.

Enhanced thermoelectric properties of n-type CoSb3-based composites by incorporating In particles

Thermoelectric devices that consist of N-and P-type TE legs in series have been well known and applied owing to the ability that could convert waste heat into electric energy. Among them, skutterudite materials have been frequently studied as the main object because they are environmentally friendly and inexpensive. In this study, N-type CoSb3-based materials were synthesized using a low-temperature synthesis method and spark plasma sintering. Those thermoelectric matrix that incorporated Indium (In) particles were prepared via manual grinding method. The thermoelectric properties, microstructures, and morphologies of the pure and In-doped Co0.91Ni0.09Sb3 samples were systematically evaluated. A maximum power factor value of 3642.3 μW/mK2 was obtained at 723 K for a Co0.91Ni0.09Sb3/1 wt%In sample and the highest figure of merit value of 1.01 was attained for a Co0.91Ni0.09Sb3/2 wt%In sample. These results indicate that Indium particles as secondary phase can improve the thermoelectric performance of N-type CoSb3-based materials.

Low temperature processed, highly stable CMOS inverter by integrating Zn-ON and tellurium thin-film transistors

Semiconductors processed at low temperature for complementary metal–oxide semiconductors (CMOS) devices are receiving considerable attention in the field of integrated electronic applications. In this work, we demonstrated a CMOS inverter constructed by n-type ZnON and p-type Te TFTs where all the processes have been done at low temperature. The electrical measurements of proposed TFTs exhibit high mobility (> 100 and > 3 cm2/Vs in case of ZnON and Te TFTs, respectively) and a stable on/off current ratio. The resulted CMOS inverter exhibits a high voltage swing and a high voltage gain of 15.89. Since all the synthesis and fabrication processes are performed at low temperatures with easy processing techniques, the results may open new opportunities in the field of integrated electronics field.

A study of interfacial electrical contact resistances of thermoelectric generators for hypersonic vehicles by experimental measurements and two-scale numerical simulations

An enormous amount of aerodynamic heat is produced on the surface of hypersonic vehicles, and the thermoelectric generator (TEG) technique holds the potential to convert aerodynamic heat into electric energy. In a TEG, the micrometer-scale roughness on TE legs and electrodes surfaces leads to the constriction of interfacial electric current and brings about electrical contact resistance (ECR). The heat transfer and conversion processes of a TEG are greatly influenced by such type of ECR. In this work, a TEG includes two ceramic substrates and a middle TE layer is established, and the TE layer consists of insulation material, N- and P-type TE legs and electrodes to connect the TE legs. The ECR and its effects on the TEG performance are studied by two-scale numerical simulations and experimental measurements. First, the practical topography of rough surfaces for TE legs and electrodes is measured, and the micrometer-scale ECR prediction numerical models are established based on the reconstructed topography. An ECR measurement platform is developed and the platform consists of a pressure device, power supply and data acquisition system which are used to provide loading pressures, voltage, and acquisition of electric current and voltage signals, respectively. The ECRs under different temperatures, loading pressures and gap mediums are then predicted and the numerical models are validated by a series of measurements. Second, the TE conversion process of TEG is simulated by a millimeter-scale model including insulation materials, TE legs, electrodes, and equivalent interfacial layers of ECR. The influence of ECR on the TE performance of TEG for the hypersonic vehicle is thus clarified.

Development of a High Perfomance Gas Thermoelectric Generator (TEG) with Possibible Use of Waste Heat

A huge concern regarding global warming, as well as the depletion of natural fuel resources, has led to a wide search for alternative energy sources. Due to their high reliability and long operation time, thermoelectric generators are of significant interest for waste heat recovery and power generation. The main disadvantage of TEGs is the low efficiency of thermoelectric commercial modules. In this work, a unique design for a multilayer TE unicouple is suggested for an operating temperature range of 50–600 °C. Two types of thermoelectric materials were selected: «low temperature» n-and p-type TE materials (for the operating temperature range of 50–300 °C) based on Bi2Te3 compounds and «middle temperature» (for the operating temperature range of 300–600 °C) n- and p-type TE materials based on the PbTe compound. The hot extrusion technology was applied to fabricate n- and p-type low-temperature TE materials. A unique design of multilayer TEG was experienced to achieve an efficiency of up to 15%. This allows for the possibility of extracting this amount of electrical power from the heat generated for domestic and water heating.

Conductometric NO2 gas sensor based on nanolayered amorphous tellurium for room temperature operation

A fast operating NO2 sensor based on amorphous Te layer of a nanometric thickness, enclosed between Pt electrodes has been developed and investigated. The gas sensor operates at room temperature and by sensitivity ~ 60% / ppm it exhibits a response time around 5 s. The long – term stability of the sensor tested by operating it 12 weeks showed no appreciable changes in the characteristics. To elucidate the mechanism of so fast gas detection, the sensor structure and active material have been investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, energy-dispersive X-ray spectroscopy (EDX) and atomic force microscopy (AFM), being followed with its characterization via studying the current - voltage characteristics, dynamic response, long – term stability, effects of temperature, humidity and other gases. It is shown that combination of used materials as well as the developed sensor design allow the simultaneous involvement of contact, and surface phenomena in sensor mechanism of operation, making possible the gas induced modulation of charge carriers simultaneously on Te film surface, accumulation regions at contacts, which include the portions of the degenerate (metallic) p-Te, as well as in contact gaps (transition regions), originated from microscopically roughness, formed since during electrode deposition. The last referred process leads to increasing the portion of the semiconducting Te nanolayer turned into metal of p-type Te, which at closed – circuit conditions results in a sharp increasing of the current flow through the device. Such explanation of obtained results reflects the fundamentals of metal – semiconductor junctions, the properties of used materials and meets the modern models proposed for interpretation of adsorption processes in similar devices.

Super-assembled atomic Ir catalysts on Te substrates with synergistic catalytic capability for Li-CO2 batteries

Rechargeable Li-CO2 batteries (LCBs) are considered as a promising candidate for the next generation energy storage system, but still be impeded by the lack of high-performance cathode catalyst and poor understanding for the complicated reaction mechanism. In the present work, we demonstrate that the catalytic capability of cathode catalyst of LCBs can be remarkably enhanced from the accelerated reaction kinetics using a p-type substrate as adsorption/desorption promoter for refined reaction route. A carbon-free atomic Ir-Te cathode catalyst with Ir atomic cluster is uniformly super-assembled on the surface of Te nanowires forming an amorphous surface layer (ca. 3 nm) to maximize the catalytic capability of active Ir sites and provide a refined reaction pathway due to synergistic effect of Ir active sites and Te substrate. The adsorption ability of Li2C2O4 during discharging and the desorption ability of CO2 species during charging of the p-type Te substrate could both promote the catalytic reaction kinetics and optimize the reaction pathways on Ir active sites. Finally, a large specific capacity of 13,247.1 mAh g−1 and an excellent high rate cyclability with stably over 350 and 200 cycles at the current density of 1000 and 2000 mA g−1 are achieved. This contribution provides a rational design strategy for high performance cathode catalyst, and intrinsic insight towards the understanding the reaction mechanisms of LCBs.

Patterning of type-II Dirac semimetal PtTe2 for optimized interface of tellurene optoelectronic device

The controllable transformation between the semiconductor and metal plays a key role for the electronic and optoelectronic applications of atomically thin two-dimensional (2D) layered materials. Herein, we report laser-driven synthesis of PtTe 2 from Pt-deposited 2D tellurium (Te) for optimized interface of 2D Te optoelectronic device. The size and shape of the synthesized area of the PtTe 2 can be designable in the laser irradiation process. The electrical properties of 2D Te change from p-type semiconducting to metallic due to the formation of semimetallic PtTe 2 after laser irradiation, increasing the conductivity by factors of 500. In addition, by using PtTe 2 contact, the carrier mobility and photoresponsivity of the 2D Te devices could be greatly enhanced. Our photodetector shows high responsivity and detectivity up to 5.8 × 10 4 A W −1 and 5.31 × 10 11 Jones, respectively. Ideal interfaces for highly performing optoelectronics devices could be realized by an original way of depositing Pt nanoparticles and patterning a semimetal compound based on the Pt at the junction. • A controllable process to synthesize and pattern a semimetallic PtTe 2 from 2D Te by laser irradiation is reported. • The electrical properties of p-type 2D Te were transformed to metallic (PtTe 2 ) with an increase of conductivity up to 500 times. • By using PtTe 2 contact, the 2D Te devices show effective enhancements of mobility and responsivity. • Ideal interfaces for highly performing optoelectronics devices could be realized by patterning a semimetal at the junction.

High-performance p-type elemental Te thermoelectric materials enabled by the synergy of carrier tuning and phonon engineering

An outstanding figure-of-merit zT ≈ 1.06 at 600 K for p-type elemental Te thermoelectrics is realized by synergistically tuning their carrier and phonon transport behaviors via a multicomponent alloying strategy.

Contact engineering high-performance ambipolar multilayer tellurium transistors

Multilayer Te nanosheets have attracted increasing attention due to their high-performance electronic transport properties and good air-stability. Theoretical simulation suggests that the electronic properties of multilayer Te nanosheets could be effectively modulated by contact engineering, but most studies have reported p-type multilayer Te devices. Here, for the first time, we report on high performance ambipolar multilayer Te filed-effect-transistors (FETs) with low work function scandium (Sc, 3.58 eV), demonstrating high mobilities of 489 and 648 cm2V−1s−1 for electron and hole transport, respectively. Multilayer Te FETs with large work function metals, such as chromium (Cr, 4.5 eV), show a typical p-type transport behavior. The band structure of multilayer Te with a small bandgap and low work function Sc result in a small contact resistance (Rc) for both of electron and hole transport, which leads to the ambipolar behavior of multilayer Te nanosheets. The ambipolar behavior of multilayer Te FETs indicates that contact engineering is a valid tool to tune the electrical properties of multilayer Te and raises the possibility of designing digital circuits based on multilayer Te.

Piezoelectric modulation of broadband photoresponse of flexible tellurium nanomesh photodetectors

Flexible photodetector shows great potential applications in intelligent wearable devices, health monitoring, and biological sensing. In this work, single crystal β-tellurium nanowires were grown on flexible muscovite by molecular beam epitaxy, constructing high-density ordered nanomesh structure. The prepared photodetectors based on tellurium nanomesh exhibit excellent mechanical flexibility, fast response in a broad range from ultraviolet to near-infrared, and good photosensitivity. We found that the flexible photodetectors with Shottky contact drastically suppressed dark current, while the response speed was lowered in comparison to the devices with ohmic contact, as holes would take a long time to tunnel through the Shottky barrier between metal and p-type Te. Moreover, the photoresponse of flexible Shottky photodetectors can be modulated by piezoelectricity of tellurium, and pronounced photocurrent increase after bending many times. Under external stress, polarization charges could tune Shottky barrier height of the metal/tellurium, resulting in variation of photocurrent. This research not only explores the broadband photoresponse and piezoelectric effect of tellurium nanomesh, but also promotes the integration and development of broadband flexible optoelectronic devices.

Galvanic exchange reaction involving Te nanowires and Ag ions for n-type Te/Ag2Te thermoelectric nanofilms

One-dimensional silver telluride nanowires (NWs) as potential thermoelectric materials have been studied extensively. At present, most of the efforts are focused on p-type silver telluride, while limited works have been achieved on the preparation of n-type silver telluride. Herein, we prepared n-type Te/Ag2Te NWs by galvanic exchange reaction and fabricated a nanofilm. As a simple and cost-effective method, galvanic exchange reaction was employed to prepare n-type Te/Ag2Te NWs with obtained Seebeck coefficient of − 93.98 μV K−1. The nanostructure and chemical composition as well as structural phase transition of as-prepared n-type Te/Ag2Te NWs were characterized and discussed. Besides, the thermoelectric thin film device consisting of n-type Te/Ag2Te nanofilms and p-type Ag-doped Te nanofilms obtained by one-step hydrothermal method was assembled and exhibited accessible output voltage as 65 mV at the 75 K temperature difference, which may provide the fundament for the development of inorganic thermoelectric nanofilm device.

Effective dopants in p-type elementary Te thermoelectrics

Bi and Sb are effective dopants in a p-type Te system and can largely enhance the thermoelectric performance of Te, the maximum ZT values of ∼0.8 in Te0.9925Bi0.0075 and ∼1.1 in Te0.985Sb0.015 at 673 K can be achieved.

Low temperature thermoelectric properties of p-type copper selenide with Ni, Te and Zn dopants

Thermoelectric figure of merit (ZT) of 0.14 at 200 K is achieved in p-type α-phase copper selenide (Cu2Se) parent compound made by a ball milling and hot pressing process which exhibits layered structure material. This figure of merit is higher than the conventional p-type TE material bismuth antimony at this temperature range. The broad maxima in the electrical resistivity data around 100 K indicates a possible charge density wave (CDW) transition and Peierls distortion, however, such kind of anomalies were not observed in other transport properties. These features suggest that p-type α-Cu2Se could be a promising thermoelectric material at low temperatures.