Phase transition and high temperature thermoelectric properties of copper selenide Cu_(2-x) Se (0 ≤ x ≤ 0.25)

With good electrical properties and an inherently complex crystal structure, Cu2-xSe is a potential ＂phonon glass electron crystal＂ thermoelectric material that has previously not attracted much interest. In this study, Cu2-xSe （0 ≤ x ≤0.25） compounds were synthesized by a melting-quenching method, and then sintered by spark plasma sintering to obtain bulk material. The effect of Cu content on the phase transition and thermoelectric properties of Cu2-xSe were investigated in the temperature range of 300 K-750 K. The results of X-ray diffraction at room temperature show that Cu2-xSe compounds possess a cubic structure with a space group of Fm3m （#225） when 0.15 〈 x ≤ 0.25, whereas they adopt a composite of monoclinic and cubic phases when 0 ≤x ≤ 0.15. The thermoelectric property measurements show that with increasing Cu content, the electrical conductivity decreases, the Seebeck coefficient increases and the thermal conductivity decreases. Due to the relatively good power factor and low thermal conductivity, the nearly stoichiometric Cu2Se compound achieves the highest ZT of 0.38 at 750 K. It is expected that the thermoelectric performance can be further optimized by doping appropriate elements and/or via a nanostructuring approach.

Single‐precursor phase‐controlled synthesis of copper selenide nanocrystals and their conversion to amorphous hollow nanostructures

The crystal phases are essential to the physicochemical properties and functionalities of materials.Copper selenide has emerged as an important and appealing semiconductor,which can exist in a variety of polymorphic phases.However,the richness of polymorphs also makes it a challenge to the direct preparation of copper selenide nanocrystals with tunable phases.Herein,two polymorphs,that is,quasitetragonal Cu2−xSe nanocubes and metastable wurtzite Cu2Se nanodisks,are successfully synthesized by using a single precursor,copper(I)selenocyanate(CuSeCN),as the Cu and Se sources.The key to phase modulation is the optimal choice of the ligand in the synthesis.The as‐prepared nanocrystals possess different morphologies and compositions,giving rise to distinct optical properties and electrical conductivities.Interestingly,the copper selenide nanocrystals can provide a platform for the rational construction of two types of amorphous hollow Au─Cu─Se nanostructures by reaction with Au(I)precursor,in which their final shapes are well kept as that of the original nanocrystal templates.This work provides an easy strategy for the phase‐controlled synthesis of copper selenide nanocrystals and enables the design of new materials for broad applications.

A simple chemical route to synthesize the umangite phase of copper selenide(Cu3Se2) thin film at room temperature

Copper selenide（Cu3Se2/thin films have been synthesized with Se as the precursor in aqueous solution by chemical bath deposition technique at room temperature.We have investigated the influence of the growth time ranging from 30 to 90 min on structural,optical and electrical properties of Cu3Se2 thin films.The as-grown film at 60 min exhibits a tetragonal structure and is（101）oriented.The maximum value of crystal size DD55 nm is attained for Cu3Se2 films grown at 60 min.The Raman spectrum reveals a pronounced peak at 259 cm 1,which is assigned to vibrational（stretching）modes from the covalent Se–Se bonds.The optical band gap energy is 1.91 to2.01 eV with growth time increased from 30 to 90 min.The scanning electron microscopy（SEM）study reveals that the grains are uniform and spread over the entire surface of the substrate of the film at 60 min.The Hall effect study reveals that the film exhibits p-type conductivity.The synthesized film showed good absorbance in the visible region which signifies that synthesized Cu3Se2 films can be suitable as a sensitized material in semiconductor sensitized solar cells.

Novel closed-cycle reaction mode for totally green production of Cu_(1.8)Se nanoparticles based on laser-generated Se colloidal solution

Non-stoichiometric copper selenide(Cu_(2-x)Se,x=0.18~0.25)nanomaterials have attracted extensive attentions due to their excellent thermoelectric,optoelectronic and photocatalytic performances.However,efficient production of Cu_(2-x)Se nanoparticles(NPs)through a green and convenient way is still hindered by the inevitable non-environmentally friendly operations in common chemical synthesis.Herein,we initially reveal the coexistence of seleninic acid content and elemental selenium(Se)NPs in pulsed laser-generated Se colloidal solution.Consequently,we put forward firstly a closedcycle reaction mode for totally green production of Cu_(1.8)Se NPs to exclude traditional requirements of high temperature and toxic precursors by using Se colloidal solution.In such closed-cycle reaction,seleninic acid works as the initiator to oxidize copper sheet to release cuprous ions which can catalyze the disproportion of Se NPs to form Se O_(3)^(2-)and Se^(2-)ions and further produce Cu_(2-x)Se NPs,and the by-product SeO_(3)^(2-)ions promote subsequent formation of cuprous from the excessive Cu sheet.In experiments,the adequate copper(Cu)sheet was simply dipped into such Se colloidal solution at 70℃,and then the stream of Cu_(1.8)SeNPs could be produced until the exhaustion of selenium source.The conversion rate of Se element reaches to more than 75%when the size of Se NPs in weakly acidic colloidal solution is limited between 1 nm and 50 nm.The laser irradiation duration shows negative correlation with the size of Se NPs and unobvious impact to the p H of the solution which both are essential to the high yield of Cu_(1.8)SeNPs.Before Cu sheet is exhausted,Se colloidal solution can be successively added without influences to the product quality and the Se conversion rate.Such green methodology positively showcases a brand-new and potential strategy for mass production of Cu_(2-x)Se nanomaterials.

Achieving thermoelectric performance of rGO/Bi_(0.5)Sb_(1.5)Te_(3)/Cu_(2)Se_(1-x)Tex composites through the scattering engineering strategy

Bismuth antimony telluride(Bi_(2–x)Sb_(x)Te_(3))is commonly used for thermoelectric generation at temperatures near ambient temperature.Here,we report incorporating reduced graphene oxide(rGO)and Cu_(2)Se_(0.98)Te_(0.02) into the Bi_(0.5)Sb_(1.5)Te_(3)(BST)(rGO/Bi_(0.5)Sb_(1.5)Te_(3-x)Cu_(2)Se_(0.98)Te_(0.02),where x=0.0%,0.1%,0.3%,0.5% and 1.0%,in mass)synthesized by a solid-state technique.The dispersion of rGO and Cu_(2)Se_(0.98)Te_(0.02) into the BST matrix improved carrier transport properties at the grain boundary interfaces and reduced thermal conductivity.Strong electron scattering at large interface barriers was responsible for increased electrical conductivity.The bulk sample of rGO/BST-0.3%Cu_(2)Se_(0.98)Te_(0.02)(in mass)possessed a low thermal conductivity of 0.76 W·m^(−1)·K^(−1) at 497 K.Enhanced phonon scattering at grain boundaries between BST and rGO/Cu_(2)Se_(0.98)Te_(0.02) caused a low thermal conductivity.At 448 K,the highest zT value for rGO/BST-0.3% Cu_(2)Se_(0.98)Te_(0.02)(in mass)was 1.64,which is 37% higher than the zT value for pure BST(zT=1.19).Results suggested that incorporating rGO and Cu_(2)Se_(0.98)Te_(0.02) into the BST matrix effectively improved thermoelectric power generation.

Biomimetic nanoparticles directly remodel immunosuppressive microenvironment for boosting glioblastoma immunotherapy

Glioblastoma(GBM),as a very aggressive cancer of central nervous system,is very challenging to completely cure by the conventional combination of surgical resection with radiotherapy and chemotherapy.The success of emerging immunotherapy in hot tumors has attracted considerable interest for the treatment of GBM,but the unique tumor immunosuppressive microenvironment(TIME)of GBM leads to the failure of immunotherapy.Here,we show the significant improvement of the immunotherapy efficacy of GBM by modulating the TIME through novel all-in-one biomimetic nanoparticles(i.e.CS-I/J@CM NPs).The nanoparticles consist of utrasmall Cu2-xSe nanoparticles(NPs)with outstanding intrinsic properties(e.g.,photo-responsive Fenton-like catalytic property for inducing immunogenic cell death(ICD)and alleviating the hypoxia of tumor),indoximod(IND,an inhibitor of indoleamine-2,3-dioxygenease in tumor),JQ1(an inhibitor for reducing the expression of PD-L1 by tumor cells),and tumor cell membrane for improving the targeting capability and accumulation of nanoparticles in tumor.We reveal that these smart CS-I/J@CM NPs could drastically activate the immune responses through remodeling TIME of GBM by multiple functions.They could(1)increase M1-phenotype macrophages at tumor site by promoting the polarization of tumor-associated macrophages through the reactive oxygen species(ROS)and oxygen generated from the Fenton-like reaction between nanoparticles and H2O2 within tumor under NIR II irradiation;(2)decrease the infiltration of Tregs cells at tumor site through the release of IND;(3)decrease the expression of PD-L1 on tumor cells through JQ1.The notable increments of anti-tumor CD8+T cells in the tumor and memory T cells(TEM)in the spleen show excellent therapy efficacy and effectively prevent the recurrence of GBM after modulation of the TIME.This work demonstrates the modulation of TIME could be a significant strategy to improve the immunotherapy of GBM and other cold tumors.

Conventional sintered Cu_(2-x)Se thermoelectric material

As the featured material of the superionic thermoelectric(TE)material family,copper-chalcogenide Cu_(2-x)Se is attracting growing research interest for its excellent TE performance derived from the satisfactory power factor and the ultra-low thermal conductivity induced by the superionic effect.Various efforts have been made and proved to be effective to further enhance the TE performance for Cu_(2-x)Se.However,this material is still far from the application stage,which is mainly due to concerns regarding control of the properties and the costly complex fabrication technology.Here we report a scalable pathway to achieve high-performance and tunable Cu_(2-x)Se,utilizing conventional sintering technology and copper(Cu)-vacancy engineering with an effective mass model.The figure of merit zT is a competitive value of 1.0 at 800 K for the optimized binary Cu_(2-x)Se,based on the precise modeling prediction and Cu-vacancy engineering.The changes in TE properties of Cu_(2-x)Se under heating-cooling cycle tests are also revealed.Our work offers the referable method along with the decent parent material for further enhancement of TE performance,paving a possible route for the application and industrialization of Cu_(2-x)Se TE materials.

Anomalous CDW ground state in Cu_(2)Se:A wave-like fluctuation of the dc I-v curve near 50 K

A charge density wave(CDW)ground state is observed in polycrystalline Cu_(2)Se below 125 K,which corresponds to an energy gap of 40.9 meV and an electron-phonon coupling constant of 0.6.Due to the polycrystalline structure,the Peierls transition process has been expanded to a wide temperature range from 90 K to 160 K.The Hall carrier concentration shows a continuous decrease from 2.1×10^(20)to 1.6×10^(20)cm^(-3)in the temperature range from 160 K to 90 K,while almost unchanged above 160 K and below 90 K.After entering the CDW ground state,a wave-like fluctuation was observed in theI-Vcurve near 50 K,which exhibits a periodic negative differential resistivity in an applied electric field due to the current.We also investigated the doping effect of Zn,Ni,and Te on the CDWground state.Both Zn and Ni doped Cu_(2)Se show a CDW character with increased energy gap and electron-phonon coupling constant,but no notable Peierls transition was observed in Te doped Cu_(2)Se.Similar wave-likeI-Vcurve was also seen in Cu_(1.98)Zn_(0.02)Se near 40 K.The regular fluctuation in the dcI-Vcurve was not magnetic field sensitive,but temperature and sample size sensitive.