Previous band structure calculations predicted Ag3AuSe2to be a semiconductor with a band gap of approximately 1 eV. Here, we report single crystal growth of Ag3AuSe2and its transport and optical properties. Single crystals of Ag3AuSe2were synthesized by slow‐cooling from the melt, and grain sizes were confirmed to be greater than 2 mm using electron backscatter diffraction. Optical and transport measurements reveal that Ag3AuSe2is a highly resistive semiconductor with a band gap and activation energy around 0.3 eV. Our first‐principles calculations show that the experimentally determined band gap lies between the predicted band gaps from GGA and hybrid functionals. We predict band inversion to be possible by applying tensile strain. The sensitivity of the gap to Ag/Au ordering, chemical substitution, and heat treatment merit further investigation.
There are only a few examples of nanocrystal synthesis with thallium (Tl). Here, we report the synthesis of uniform, ligand‐stabilized colloidal nanocrystals of TlBr and Tl2AgBr3nanocrystals with average diameter ranging between 10 and 20 nm. TlBr nanocrystals are made by hot injection of trimethylsilyl bromide (TMSBr) into solutions of oleylamine, oleic acid and octadecene with thallium (III) or thallium (I) acetate. Tl2AgBr3nanocrystals form when silver (I) acetate is included in the reaction. The TlBr nanocrystals have CsCl crystal structure with a direct band gap of 3.1 eV. The Tl2AgBr3nanocrystals have trigonal dolomite crystal structure with an indirect band gap of 3.1 eV. The TlBr nanocrystals made with thallium (III) were sufficiently uniform to assemble into face‐centered cubic (fcc) superlattices.
more » « less- NSF-PAR ID:
- 10148916
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- ChemNanoMat
- Volume:
- 6
- Issue:
- 5
- ISSN:
- 2199-692X
- Page Range / eLocation ID:
- p. 790-796
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
Abstract -
Chromium-doped SrTiO 3 nanocrystals of perovskite structure type and 45 nm (±15 nm) edge lengths were obtained by hydrothermal synthesis in water from titanium oxide, strontium hydroxide, and chromium( iii ) nitrate. According to XPS, the majority of the surface chromium (68.3%) is present in the 3+ state and the remainder (32.2%) in the 6+ state. Optical spectroscopy confirms a broad absorption at 2.3–2.9 eV from Cr(3+) dopant states, in addition to the 3.2 eV band edge of the SrTiO 3 host. After modification with Pt nanoparticles, Cr-doped SrTiO 3 nanocrystals catalyze photochemical H 2 evolution from aqueous methanol under visible light illumination (>400 nm) and with an apparent quantum yield of 0.66% at 435 nm. According to surface photovoltage spectroscopy (SPS), Cr-doped SrTiO 3 nanocrystals deposited onto gold substrates are n-type and have an effective band gap of 1.75 eV. SPS and transient illumination experiments at 2.50 eV reveal an anomalous surface photovoltage that increases with prior light exposure to values of up to −6.3 V. This photovoltage is assigned to ferroelectric polarization of the material in the space charge layer at the Au/SrTiO 3 :Cr interface. The polarization is stable for 24 h in vacuum but disappears after 12 h when samples are stored in air. The electric polarizability of SrTiO 3 :Cr is confirmed when films are exposed to static electric fields (1.20 MV m −1 ) in a fixed capacitor configuration. The discovery of a ferroelectric effect in Cr-doped SrTiO 3 could be significant for the development of improved photocatalysts for the conversion of solar energy into fuel.more » « less
-
Abstract The synthesis of BaZr(S,Se)3chalcogenide perovskite alloys is demonstrated by selenization of BaZrS3thin films. The anion‐exchange process produces films with tunable composition and band gap without changing the orthorhombic perovskite crystal structure or the film microstructure. The direct band gap is tunable between 1.5 and 1.9 eV. The alloy films made in this way feature one‐hundred‐times stronger photoconductive response and a lower density of extended defects, compared to alloy films made by direct growth. The perovskite structure is stable in high‐selenium‐content thin films with and without epitaxy. The manufacturing‐compatible process of selenization in H2Se gas may spur the development of chalcogenide perovskite solar cell technology.
-
Abstract Above‐band gap optical excitation of non‐centrosymmetric semiconductors can lead to the spatial shift of the center of electron charge in a process known as shift current. Shift current is investigated in single‐crystal SnS2, a layered semiconductor with the band gap of ≈2.3 eV, by THz emission spectroscopy and first principles density functional theory (DFT). It is observed that normal incidence excitation with above gap (400 nm; 3.1 eV) pulses results in THz emission from 2H SnS2() polytype, where such emission is nominally forbidden by symmetry. It is argued that the underlying symmetry breaking arises due to the presence of stacking faults that are known to be ubiquitous in SnS2single crystals and construct a possible structural model of a stacking fault with symmetry properties consistent with the experimental observations. In addition to shift current, it is observed THz emission by optical rectification excited by below band gap (800 nm; 1.55 eV) pulses but it requires excitation fluence more than two orders of magnitude higher to produce same signal amplitude. These results suggest that ultrafast shift current in which can be excited with visible light in blue–green portion of the spectrum makes SnS2a promising source material for THz photonics.
-
null (Ed.)CrBr 3 is a layered van der Waals material with magnetic ordering down to the 2D limit. For decades, based on optical measurements, it is believed that the energy gap of CrBr 3 is in the range of 1.68–2.1 eV. However, controversial results have indicated that the band gap of CrBr 3 is possibly smaller than that. An unambiguous determination of the energy gap is critical to the correct interpretations of the experimental results of CrBr 3 . Here, we present the scanning tunneling microscopy and spectroscopy (STM/S) results of CrBr 3 thin and thick flakes exfoliated onto highly ordered pyrolytic graphite (HOPG) surfaces and density functional theory (DFT) calculations to reveal the small energy gap (peak-to-peak energy gap to be 0.57 ± 0.04 eV; or the onset signal energy gap to be 0.29 ± 0.05 eV from d I /d V spectra). Atomic resolution topography images show the defect-free crystal structure and the d I /d V spectra exhibit multiple peak features measured at 77 K. The conduction band – valence band peak pairs in the multi-peak d I /d V spectrum agree very well with all reported optical transitions. STM topography images of mono- and bi-layer CrBr 3 flakes exhibit edge degradation due to short air exposure (∼15 min) during sample transfer. The unambiguously determined small energy gap settles the controversy and is the key in better understanding CrBr 3 and similar materials.more » « less