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Gaiser, Christof; Zandt, Thorsten; Krapf, A.; Serverin, Ralf; Janowitz, C.; Manzke, R.

doi:10.1103/physrevb.69.075205

Cited by 58 publications

(30 citation statements)

References 13 publications

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“…Significantly larger band gap values determined from MR mean that HfS 2 and HfSe 2 are indirect gap semiconductors. The found indirect gaps are consistent with previous absorption studies for these crystals 47 , see Table 1. Extended optical studies will be reported elsewhere.…”

Section: Resultssupporting

confidence: 93%

Photoacoustic and modulated reflectance studies of indirect and direct band gap in van der Waals crystals

Zelewski

Kudrawiec

2017

Sci Rep

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Photoacoustic (PA) and modulated reflectance (MR) spectroscopy have been applied to study the indirect and direct band gap for van der Waals (vdW) crystals: dichalcogenides (MoS 2 , MoSe 2 , MoTe 2 , HfS 2 , HfSe 2 , WS 2 , WSe 2 , ReS 2 , ReSe 2 , SnS 2 and SnSe 2 ) and monochalcogenides (GaS, GaSe, InSe, GeS, and GeSe). It is shown that the indirect band gap can be determined by PA technique while the direct band gap can be probed by MR spectroscopy which is not sensitive to indirect optical transitions. By measuring PA and MR spectra for a given compound and comparing them with each other it is easy to conclude about the band gap character in the investigated compound and the energy difference between indirect and direct band gap. In this work such measurements, comparisons, and analyses have been performed and chemical trends in variation of indirect and direct band gap with the change in atom sizes have been discussed for proper sets of vdW crystals. It is shown that both indirect and direct band gap in vdW crystals follow the well-known chemical trends in semiconductor compounds.Van der Waals (vdW) crystals are known for a long time 1-14 but in recent years some of them have gained significant interest because of unique mechanical and optical properties of samples obtained by exfoliation of bulk material to atomically thin layers [15][16][17][18][19][20][21][22][23][24][25] . Since pioneering reports on optical properties of MoS 2 layers 15,16 , it is well established that the electronic band structure of MoS 2 strongly varies with the number of layers and exhibits indirect-to-direct band gap transition with the size reduction to a single layer. Similar studies of optical properties and the electronic band structure have been performed on other van der Waals crystals such as MoSe 2 , MoTe 2 , WS 2 , and WSe 2 22-25 . Some of them were investigated quite recently as bulk materials 13,14 but interest in these materials was low due to no potential applications in semiconductor devices. At this moment the situation is different. VdW crystals are exciting because of interesting physics of two-dimensional (2D) layers and their potential applications in novel semiconductor devices [26][27][28] . Because of this even bulk van der Waals crystals are interesting to study since many of them are not explored experimentally. One method which has never been applied to study most of van der Waals crystals is photoacoustic (PA) spectroscopy. We show that this technique is an excellent tool to study the band gap in van der Waals crystals since it is sensitive to indirect gap transitions which cannot be observed in photoluminescence. In combination with modulated reflectance (MR), which is known as a state of the art absorption-like technique to study direct optical transitions 29 , it is possible to conclude about the band gap character in van der Waals crystals and the spectral separation between the indirect and direct gap. In this work we applied PA and MR to study indirect and direct gap in many van der Waals crystals ...

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Section: Resultssupporting

confidence: 93%

Photoacoustic and modulated reflectance studies of indirect and direct band gap in van der Waals crystals

Zelewski

Kudrawiec

2017

Sci Rep

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“…5(c)) employing Kraut's method 20 and experimental literature values for the energy gaps. 9,13 The energy position of the Fermi level is compatible with the measured VBM and workfunction values of the two materials and they are compatible with electron affinity rule except from a vacuum level mismatch d vac ¼ 0.33 eV which may be partly due to the experimental uncertainties in measuring the MoSe 2 WF and the energy position of E F . The band alignment results between HfSe 2 and MoSe 2 in Fig.…”

supporting

confidence: 58%

“…Other 2D semiconductors 4 as, for example, group IVB metal dichalcogenides ((Zr,Hf)X 2 ) 5 with a 1T crystal structure and potentially better properties need to be investigated. HfSe 2 , in particular, is predicted [6][7][8] to be an indirect gap semiconductor with a measured bulk value of Eg $ 1.1 eV, similar to Si, 9,10 and a remarkably high phonon-limited mobility of $3500 cm 2 /VÁs at room temperature. 11 The existence of a large number of 2D materials with similar crystal structure but with diverse electronic band structure properties including energy gap, electron affinities, workfunction (WF), etc., allows for the formation of high quality van der Waals heteroepitaxial materials combinations with abrupt interfaces and a variety of band alignments offering flexibility for the design of (opto)electronic devices.…”

mentioning

confidence: 99%

Two-dimensional semiconductor HfSe2 and MoSe2/HfSe2 van der Waals heterostructures by molecular beam epitaxy

Aretouli

Tsipas

Tsoutsou

et al. 2015

Applied Physics Letters

127

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Using molecular beam epitaxy, atomically thin 2D semiconductor HfSe2 and MoSe2/HfSe2 van der Waals heterostructures are grown on AlN(0001)/Si(111) substrates. Details of the electronic band structure of HfSe2 are imaged by in-situ angle resolved photoelectron spectroscopy indicating a high quality epitaxial layer. High-resolution surface tunneling microscopy supported by first principles calculations provides evidence of an ordered Se adlayer, which may be responsible for a reduction of the measured workfunction of HfSe2 compared to theoretical predictions. The latter reduction minimizes the workfunction difference between the HfSe2 and MoSe2 layers resulting in a small valence band offset of only 0.13 eV at the MoSe2/HfSe2 heterointerface and a weak type II band alignment.

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“…not unambiguously determine the transition mechanism for fundamental absorption for bulk materials with 2D densities of states. 9,30,31 We modeled the absorbance assuming direct-allowed, direct-forbidden, indirect-allowed and indirect-forbidden gaps using both 2D and 3D densities of states (Figure 7). All of these plots estimated fundamental gaps ranging from 1.48 to 1.60 eV.…”

Section: Chapter 2: Air-stabilitymentioning

confidence: 99%

Stability and Exfoliation of Germanane: A Germanium Graphane Analogue

et al. 2013

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Graphene's success has shown that it is not only possible to create stable, single-atom thick sheets from a crystalline solid, but that these materials have fundamentally different properties than the parent material. We have synthesized for the first time, mm-scale crystals of a hydrogen-terminated germanium multilayered graphane analogue (germanane, GeH) from the topochemical deintercalation of CaGe2. This layered van der Waals solid is analogous to multilayered graphane (CH). The surface layer of GeH only slowly oxidizes in air over the span of 5 months, while the underlying layers are resilient to oxidation based on X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FTIR) measurements. The GeH is thermally stable up to 75 o C, however, above this temperature amorphization and dehydrogenation begin to occur. These sheets can be mechanically exfoliated as single and few layers onto SiO2/Si surfaces. This material represents a new class of covalently terminated graphane analogues and has great potential for a wide range of optoelectronic and sensing applications, especially since theory predicts a direct band gap of 1.53 eV and an electron mobility ~five times higher than that of bulk Ge.iii Acknowledgements

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Band-gap engineering withHfSxSe2−x

Cited by 58 publications

References 13 publications

Photoacoustic and modulated reflectance studies of indirect and direct band gap in van der Waals crystals

Photoacoustic and modulated reflectance studies of indirect and direct band gap in van der Waals crystals

Two-dimensional semiconductor HfSe2 and MoSe2/HfSe2 van der Waals heterostructures by molecular beam epitaxy

Stability and Exfoliation of Germanane: A Germanium Graphane Analogue

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