Cu₂Se Nanoparticles with Tunable Electronic Properties Due to a Controlled Solid-State Phase Transition Driven by Copper Oxidation and Cationic Conduction

Abstract

Stoichiometric copper(I) selenide nanoparticles have been synthesized using the hot injection method. The effects of air exposure on the surface composition, crystal structure, and electronic properties were monitored using X-ray photoelectron spectroscopy, X-ray diffraction, and conductivity measurements. The current-voltage response changes from semiconducting to ohmic, and within a week a 3000-fold increase in conductivity is observed under ambient conditions. The enhanced electronic properties can be explained by the oxidation of Cu(+) and Se(2-) on the nanoparticle surface, ultimately leading to a solid-state conversion of the core from monoclinic Cu(2)Se to cubic Cu(1.8)Se. This behavior is a result of the facile solid-state ionic conductivity of cationic Cu within the crystal and the high susceptibility of the nanoparticle surface to oxidation. This regulated transformation is appealing as one could envision using layers of Cu(2)Se nanoparticles as both semiconducting and conducting domains in optoelectronic devices simply by tuning the electronic properties for each layer through controlled oxidation.

Promising bulk nanostructured Cu2Se thermoelectrics via high throughput and rapid chemical synthesis

“…Using the empirical relation for the band gap, E g =2eS max T max , and our Seebeck coefficient data [24] , the band gap energy is estimated to be 0.72 eV for the sample Cu 2 Se_Nano_nl, and 0.83 eV for the sample Cu 2 Se_Nano_l. Both values are relatively lower than the band gap energy of nano-Cu 2 Se reported by Riha et al [25] . The wide 11 range in observed band gap energies and different electronic behaviors in our samples compared to the literature might be due to differences in the Cu to Se stoichiometry, large grain size distributions, and grain size effects in the compound [26] .…”

Aqueous preparation of surfactant-free copper selenide nanowires

“…[1][2][3][4] For example, both bulk and nano-scale copper selenides, despite their simple chemical formula, have very complicated crystal structures (e.g. tetragonal-, orthorhombic-, and cubic structures) and variable composition (Cu 2-x Se, 0≤x≤1), which offer an incredible wealth of properties for diverse applications such as thermoelectric conversion, lithium-ion battery, photocatalysis, quantum-dot-sensitized solar cells, and photoacoustic imaging.…”

“…An example is the cuprous selenide (Cu 2 Se) nanoparticles, which are easily oxidized into non-stoichiometric Cu 1.8 Se, and become into surprisingly good p-type semiconductor with over 3000 times of enhancements in electrical conductivity. [2] In order to tune the properties of nano-scale copper selenides, a number of techniques and strategies, such as hydro-or solvo-thermal approach, [2,15,16] sonochemistry, [17] electrochemical-deposition, [18] microwave-assistant route, [19] ball milling technique, [6] and chemical welding method, [5] have been developed to prepare nanocrystals, [20] nanotubes/wires, [15,[21][22][23] nanocages, [24] dendrimers, [16,25] and nanosheets [26] with well-defined size, morphology, crystal structure, and composition.…”

Highly efficient photoanodes based on cascade structural semiconductors of Cu2Se/CdSe/TiO2: a multifaceted approach to achieving microstructural and compositional control

“…Transition-metal chalcogenides semiconductors such as CdS, 27,28 Cu 2 S, 29 CuInS 2 , 30 CdSe, [31][32][33][34] Cu 2 Se, [35][36][37][38][39] Cu 2 ZnSnS 4 40 and heterojunction research of CdSe/TiO 2 NTAs focuses mainly on the formation of single-junction nano-materials, which generally use co-sensitization structure has demonstrated greater efficiency than the corresponding single-junction photoanodes. 43,44 Merging the 4 multilayered narrow-band-gap and the TiO 2 large-band-gap semiconductors can result in the formation of multi-heterostructures and 5 produce a novel photocatalyst with continuously changed band gap, thereby effectively enhancing the photoresponse.…”

“…So far there are several interesting 7 reports on using Cu 2 Se as an absorber material in photovoltaic devices. [35][36][37][38][39]45 A widely varying range of band gap energy for Cu 2 Se 8 from indirect band gap of 1.1-1.5 eV to direct band gap of 2.0-2.3 eV have been reported. The ability to tailor the band gap energy is 9 directly related to its capability of microscopic controls in composition and structure.…”