The direct-to-indirect band gap crossover in two-dimensional van der Waals Indium Selenide crystals

Mudd, Garry W.; Molas, Maciej R.; Chen, X.; Zólyomi, Viktor; Nogajewski, Karol; Kudrynskyi, Z. R.; Kovalyuk, Z. D.; Yusa, Go; Makarovsky, O.; Eaves, L.; Potemski, M.; Fal’ko, Vladimir I.; Patanè, A.

doi:10.1038/srep39619

Cited by 166 publications

(182 citation statements)

References 34 publications

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“…This conclusion is also confirmed by photoluminescence (PL) measurements since a bandgap-related emission was observed for both samples. It is worth noting that PL for such crystals was reported by other authors, see for example refs 52–55 , and therefore emission properties are not further discussed in this work. In addition, the MR spectrum observed for InSe is very consistent with previous photoreflectance studies of γ-InSe 56 .…”

Section: Resultsmentioning

confidence: 84%

“…In addition, the MR spectrum observed for InSe is very consistent with previous photoreflectance studies of γ-InSe 56 . The bulk InSe sample studied in this work is also γ-polytype (i.e., 2H polytype according to the nomenclature used in this work) and extended studies on exfoliated layers from such crystal can be found in refs 54,55 . The oscillating signal below the band gap is due to F-P oscillations with a short period.…”

Section: Resultsmentioning

confidence: 99%

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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: Resultsmentioning

confidence: 84%

Section: Resultsmentioning

confidence: 99%

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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“…This is based on observations of their optical properties, which can differ strongly from those of their parent bulk materials, and which have demonstrated room-temperature electroluminesence 28 , strong photoresponsivity 23 with a broad spectral response 26,27,29 , and band gap tunability 24 . Recent studies of luminescence 30 and magnetoluminescence 31 in InSe have shown a strong dependence of the band gap on the number of layers, from ∼ 2.8 eV for the monolayer to ∼ 1.3 eV for thick films. These experiments have identified two main photoluminescence lines, interpreted 30 as a lower energy transition between bands dominated by s and p z orbitals (A-line) and hot luminescence, involving holes in a deeper valence band based on p x and p y orbitals (B-line).…”

Section: Introductionmentioning

confidence: 99%

Spin-orbit coupling, optical transitions, and spin pumping in monolayer and few-layer InSe

2017

Self Cite

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We show that spin-orbit coupling (SOC) in InSe enables the optical transition across the principal band gap to couple with in-plane polarized light. This transition, enabled by px,y ↔ pz hybridization due to intra-atomic SOC in both In and Se, can be viewed as a transition between two dominantly s-and pz-orbital based bands, accompanied by an electron spin-flip. Having parametrized k · p theory using first principles density functional theory we estimate the absorption for σ ± circularly polarized photons in the monolayer as ∼ 1.5%, which saturates to ∼ 0.3% in thicker films (3 − 5 layers). Circularly polarized light can be used to selectively excite electrons into spin-polarized states in the conduction band, which permits optical pumping of the spin polarization of In nuclei through the hyperfine interaction.

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“…The thicknesses of the encapsulated InSe flakes range from 20 nm to 2.4 nm, corresponding to around 24 atomic layers to 3 layers [8]. The thickness was measured using atomic force microscopy, as shown in figure 1b.…”

Section: Sample and Methodsmentioning

confidence: 99%

Photoluminescence dynamics in few-layer InSe

Venanzi

Arora

Winnerl

et al. 2020

Phys. Rev. Materials

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We study the optical properties of thin flakes of InSe encapsulated in hBN. More specifically, we investigate the photoluminescence (PL) emission and its dependence on sample thickness and temperature. Through the analysis of the PL lineshape, we discuss the relative weights of the exciton and electron-hole contributions. Thereafter we investigate the PL dynamics. Two contributions are distinguishable at low temperature: direct bandgap electron-hole and defect-assisted recombination. The two recombination processes have lifetime of τ1 ∼ 8 ns and τ2 ∼ 100 ns, respectively. The relative weights of the direct bandgap and defect-assisted contributions show a strong layer dependence due to the direct-to-indirect bandgap crossover. Electron-hole PL lifetime is limited by population transfer to lower-energy states and no dependence on the number of layers was observed. The lifetime of the defect-assisted recombination gets longer for thinner samples. Finally, we show that the PL lifetime decreases at high temperatures as a consequence of more efficient non-radiative recombinations.

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The direct-to-indirect band gap crossover in two-dimensional van der Waals Indium Selenide crystals

Cited by 166 publications

References 34 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

Spin-orbit coupling, optical transitions, and spin pumping in monolayer and few-layer InSe

Photoluminescence dynamics in few-layer InSe

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