Ultrafast dynamics in van der Waals heterostructures

Abstract

Van der Waals heterostructures are synthetic quantum materials composed of stacks of atomically thin two-dimensional (2D) layers. Because the electrons in the atomically thin 2D layers are exposed to layer-layer coupling, the properties of van der Waals heterostructures are defined not only by the constituent monolayers, but also by the interactions between the layers. Many fascinating electrical, optical, and magnetic properties have recently been reported in different types of van der Waals heterostructures. In this review we focus on unique excited-state dynamics in transition metal dichalcogenide (TMDC) heterostructures. TMDC monolayers are the most widely studied 2D semiconductors, featuring prominent exciton states and accessibility to the valley degree of freedom. Many TMDC heterostructures are characterized by a staggered band alignment. This band alignment has profound effects on the evolution of the excited states in heterostructures, including ultrafast charge transfer between the layers, the formation of interlayer excitons, and the existence of longlived spin and valley polarization in resident carriers. Here we review recent experimental and theoretical efforts to elucidate electron dynamics in TMDC heterostructures, extending from time scales of femtoseconds to microseconds, and comment on the relevance of these effects for potential applications in optoelectronic and valleytronic/spintronic devices. Main text Advances in the isolation and manipulation of atomically-thin sheets of two-dimensional (2D) crystals, starting with the investigations of graphene a decade ago, have ushered in a new era of basic scientific research and technological innovation. 2D layers with diverse properties can now be prepared separately and then stacked together to form new types of quantum materials, known as van der Waals (vdW) heterostructures. The ability to combine materials with monolayer precision enables the design and creation of functional 2D materials that do not exist in nature. Today we have at our disposal a wide variety of atomically thin 2D layers, ranging from semiconducting MoS 2 and insulating hexagonal boron-nitride (h-BN) to magnetic CrI 3 and superconducting NbSe 2 , that can be stacked one upon the other. Since the electrons in atomically thin layers are exposed, different quantum states found in the individual layers can interact and couple to one another in ways that are not possible in other systems. VdW heterostructures constitute a vast family of new quantum materials, since they are defined not only by the combination of constituent monolayer materials, but also by the stacking sequence and relative crystallographic alignment of the layers. Further control of physical properties in 2D vdW heterostructures can be achieved through the application of electrostatic gating and fields, as well as substrate and strain engineering. Many fascinating physical phenomena have been reported in different vdW heterostructures, as exemplified by transport measurements revealing Hofstadter butterfly states,...

2D Metal Chalcogenides for IR Photodetection

“…Heterostructure is an important cornerstone of modern electronic devices. The heterostructures not only show outstanding photoelectric properties but also have high degree of adjustment due to energy band alignments . The continuous development of 2DMCs has further given rise to the research of hybrid heterostructures IR photodetection.…”

Band nesting and exciton spectrum in monolayer MoS2

“…There is currently great interest in van der Waals materials, including semiconductors, topological insulators, semimetals, superconductors and ferromagnets [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] . The weak bonding of atomic layers in bulk materials allows to peel off single layers from the bulk and reassemble them into new combinations not found in nature 3 .…”

Diabatic Hamiltonian construction in van der Waals heterostructure complexes

“…Photo-induced charge separation/transfer processes widely exist in van der Waals (vdW) heterostructure complexes. [1][2][3][4] Due to their importance, considerable efforts were made to investigate the photoinduced charge transfer dynamics of these materials from the theoretical point of view. 3,[5][6][7][8][9][10][11] For example, transition-metal dichalcoge nide (TMD) van der Waals heterostructures MoS2/WS2 attracted tremendous interests due to their potential applicability as optoelectronic and solar energy conversion devices.…”

“…[1][2][3][4] Due to their importance, considerable efforts were made to investigate the photoinduced charge transfer dynamics of these materials from the theoretical point of view. 3,[5][6][7][8][9][10][11] For example, transition-metal dichalcoge nide (TMD) van der Waals heterostructures MoS2/WS2 attracted tremendous interests due to their potential applicability as optoelectronic and solar energy conversion devices. [1][2][3][4][12][13][14][15] By using photoluminescence and femtosecond pump-probe measurements, Hong et al 12 found that the hole transfer in MoS2/WS2 was at the femtosecond scale (50 fs), indicating it is a promising candidate for optoelectronic and photovoltaic applicatio ns.…”