Nanomechanical probing of the layer/substrate interface of an exfoliated InSe sheet on sapphire

Abstract

Van der Waals (vdW) layered crystals and heterostructures have attracted substantial interest for potential applications in a wide range of emerging technologies. An important, but often overlooked, consideration in the development of implementable devices is phonon transport through the structure interfaces. Here we report on the interface properties of exfoliated InSe on a sapphire substrate. We use a picosecond acoustic technique to probe the phonon resonances in the InSe vdW layered crystal. Analysis of the nanomechanics indicates that the InSe is mechanically decoupled from the substrate and thus presents an elastically imperfect interface. A high degree of phonon isolation at the interface points toward applications in thermoelectric devices, or the inclusion of an acoustic transition layer in device design. These findings demonstrate basic properties of layered structures and so illustrate the usefulness of nanomechanical probing in nanolayer/nanolayer or nanolayer/substrate interface tuning in vdW heterostructures.

Coherent acoustic phonons in van der Waals nanolayers and heterostructures

“…The present work focuses on elastic properties of interfaces rather than phonon properties of the layers, as reported in earlier works [55][56][57][58]. Furthermore, our conclusions are broader than those in the earlier work by Beardsley et al [58], which reported InSe layers weakly coupled to a substrate.…”

“…The present work focuses on elastic properties of interfaces rather than phonon properties of the layers, as reported in earlier works [55][56][57][58]. Furthermore, our conclusions are broader than those in the earlier work by Beardsley et al [58], which reported InSe layers weakly coupled to a substrate. In the present work, we show that the elastic coupling of a vdW nanolayer to its supporting substrate may vary and can be described by the spring model; also, in both InSe/InSe and InSe/hBN vdW heterostructures, we observe a strong elastic coupling, well described by the acoustic mismatch model.…”

“…Also, due to strong elastic anisotropy and phonon quantization, 2D vdW layers could provide an ideal system for phonon nanoscopy and nondestructive sensitive imaging of molecules and cells coupled to the nanolayers by vdW forces. Although these research areas are still in their infancy, recent experimental works have demonstrated the possibility of generating coherent phonons in vdW nanolayers, including graphene [6], WSe 2 [55], WTe 2 [56], MoS 2 [57], and InSe [58].…”

Time-Domain Investigations of Coherent Phonons in van der Waals Thin Films

“…They are characterized by an extracted frequency in the GHz range, which no longer depends on the probe wavelength, but increases when the material gets thinner, as shown in Figure 7 c,d. This has been observed in exfoliated few layer crystals of WSe 2 [ 133 ], MoS 2 [ 143 ], MoSe 2 [ 132 ], InSe [ 135 , 136 , 137 ], PtSe 2 [ 134 ], BP [ 126 ], Bi 2 Se 3 [ 243 ], and Bi 2 Te 3 [ 139 ]. In all these examples, the penetration depth of the pump beam gets larger than the thickness of the sample.…”

“…The hypothesis in early studies was to assume a node when the layer is supported. This could lead to wrong assignment and error in the values of extracted parameters (see [ 143 ], commented in [ 135 ] and Section 4.3 ). It was later acknowledged that a bad microscopic contact between the deposited vdW crystal and the substrate could induce an effective decoupling of the mechanical vibrations, and the presence of an anti-node at the interface [ 133 , 135 ].…”