Narrow-band-gap IV-VI semiconductors offer promising optoelectronic properties for integration as light-absorbing components in field-effect transistors, photodetectors, and photovoltaic devices. Importantly, colloidal nanostructures of these materials have the potential to substantially decrease the fabrication cost of solar cells because of their ability to be solution-processed. While colloidal nanomaterials formed from IV-VI lead chalcogenides such as PbS and PbSe have been extensively investigated, those of the layered semiconductors SnS, SnSe, GeS, and GeSe have only recently been considered. In particular, there have been very few studies of the germanium chalcogenides, which have band-gap energies that overlap well with the solar spectrum. Here we report the first synthesis of colloidal GeS and GeSe nanostructures obtained by heating GeI(4), hexamethyldisilazane, oleylamine, oleic acid, and dodecanethiol or trioctylphosphine selenide to 320 °C for 24 h. These materials, which were characterized by TEM, SAED, SEM, AFM, XRD, diffuse reflectance spectroscopy, and I-V conductivity measurements, preferentially adopt a two-dimensional single-crystal nanosheet morphology that produces fully [100]-oriented films upon drop-casting. Optical measurements indicated indirect band gaps of 1.58 and 1.14 eV for GeS and GeSe, respectively, and electrical measurements showed that drop-cast films of GeSe exhibit p-type conductivity.

Single-crystal colloidal nanosheets of GeS and GeSe.

GeSe is a narrow band gap IV–VI semiconductor that has been attracting increasing attention as a potential alternative material for photovoltaics, along with other optical and electrical applications. However, unlike several other narrow band gap chalcogenide semiconductors, very few examples of GeSe nanostructures have been reported. One-dimensional nanostructures are particularly attractive, because they can serve as building blocks for nanostructured electronic devices. As a step toward both increasing the morphological diversity of GeSe nanomaterials and expanding the library of electronic materials that are accessible as one-dimensional nanostructures, we report here the colloidal synthesis and electrical properties of GeSe nanobelts. The GeSe nanobelts were synthesized by first heating a one-pot reaction mixture of GeI4, TOP-Se, oleylamine, oleic acid, and hexamethyldisilazane to 320 °C, then adding additional TOP-Se and heating for several additional hours. Aliquot studies revealed that an amorphou...

Colloidal synthesis and electrical properties of GeSe nanobelts

GeSe is a IV-VI semiconductor, like the excellent thermoelectric materials PbTe and SnSe. Orthorhombic GeSe has been predicted theoretically to have good thermoelectric performance but is difficult to dope experimentally. Like PbTe, rhombohedral GeTe has a multivalley band structure, which is ideal for thermoelectrics and also promotes the formation of Ge vacancies to provide enough carriers for electrical transport. Herein, we investigate the thermoelectric properties of GeSe alloyed with AgSbSe2, which stabilizes a new rhombohedral structure with higher symmetry that leads to a multivalley Fermi surface and a dramatic increase in carrier concentration. The zT of GeAg0.2Sb0.2Se1.4 reaches 0.86 at 710 K, which is 18 times higher than that of pristine GeSe and over four times higher than doped orthorhombic GeSe. Our results open a new avenue towards developing novel thermoelectric materials via crystal phase engineering using a strategy of entropy stabilization of high symmetry alloys.

High Thermoelectric Performance of New Rhombohedral Phase of GeSe stabilized through Alloying with AgSbSe2

We have performed an ab initio calculation of the germanium selenide electronic structure, adopting the LDA and GGA approximations for the exchange-correlation potential within the DFT. These calculations have been carried out with and without the inclusion of the spin-orbit interaction. The subtle changes it produces in the band structure, the density of states and the optical properties have been discussed. Also, we propose the s-Ge state contribution at the edge of the valence band as having an important role. Based on our electronic structure, we discuss germanium selenide experimental core spectra and optical properties. We found excellent agreement between our results and available experimental core spectra data, and our calculated optical functions of GeSe explain the origin of the optical transitions, comparing them satisfactorily against existing experimental data.

https://iopscience.iop.org/article/10.1088/0953-8984/19/18/186211/pdf

Ab initio calculations of the electronic and optical properties of germanium selenide.

The van der Waals material GeSe is a potential solar absorber, but its optoelectronic properties are not yet fully understood. Here, through a combined theoretical and experimental approach, the op...

GeSe: Optical Spectroscopy and Theoretical Study of a van der Waals Solar Absorber

The absorption coefficient of GeSe is determined from the measured values of transmissivity and reflectivity. The measurements are carried out with plane polarized light with the plane of polarization parallel to the a- and c-axes which lie in the cleavage plane in the temperature range from room temperature to near liquid nitrogen temperature. Near the fundamental absorption edge the results are fitted to an equation corresponding to an indirect forbidden transition with a single type of phonon engaged in the interband absorption process. The optical energy gap at room temperature is found to be (1.075 ± 0.017) and (1.080 ± 0.016) eV for the a- and c-axes, respectively. The dependence of the energy gap on temperature is linear in the temperature range from 96 to 250 K with a negative temperature coefficient dEg/dT equal to −0.43 and −0.49 meV/K for the a- and c-axis, respectively.



Der Absorptionskoeffizient von GeSe wird aus den gemessenen Werten der Transmission und des Reflexionsvermogens bestimmt. Die Messungen werden im Bereich von Zimmertemperatur bis zu Temperaturen in der Nahe des flussigen Stickstoffs mit planpolarisiertem Licht und Polarisationsebene parallel zur a- und c-Achse, die in Spaltebene liegen, durchgefuhrt. In der Nahe der Grundgitterabsorptionskante werden die Ergebnisse an eine Gleichung angepast, die einem verbotenen indirekten Ubergang mit einem Phononentyp, der am Interbandabsorptionsprozes beteiligt ist, entspricht. Die optische Energiebandlucke bei Zimmertemperatur wird zu (1,075 ± 0,017) und (1,080 ± 0,016) eV fur die a- bzw. c-Achse bestimmt. Die Abhangigkeit der Bandlucke von der Temperatur ist im Temperaturbereich von 96 bis 250 K linear mit einem negativen Temperatur-koeffizienten dEg/dT von −0,43 und −0,49 meV/K fur die a- bzw. c-Achse.

A. M. Elkorashy

Papers

Indirect forbidden fundamental absorption edge in germanium selenide single crystals