Tailoring optical and electrical characteristics of layered materials through van der Waals heterojunctions

Abstract

The feasibility of isolation of layered materials and arbitrary stacking of different materials provide plenty of opportunities to realize van der Waals heterostructures (vdWhs) with desired characteristics. In this thesis, we experimentally demonstrate the tunability of optical and electrical characteristics of transition metal dichalcogenides (TMDs), a class of layered materials, using their vdWhs. Monolayer (1L) TMDs exhibit remarkable light-matter interaction by hosting direct bandgap, strongly bound excitonic complexes, ultra-fast radiative decay, many-body states, and coupled spin-valley degrees of freedom. However, their sub-nm thickness limits light absorption, impairing their viability in photonic and optoelectronic applications.

The physical proximity of layers in vdWhs drives strong interlayer dipole-dipole coupling resulting in nonradiative energy transfer (NRET) from one layer (donor) to another (acceptor) under spectral resonance. Motivated by the high efficiency of NRET in vdWhs, we study the prospect of enhancement of optical properties of a 1L-TMD stacked on top of strongly absorbing, non-luminescent, multilayer SnSe2 whose direct bandgap is close to exciton emission of 1L-TMDs – MoS2 and WS2. We show that NRET enhances both single-photon and two-photon luminescence by one order of magnitude in such vdWhs. We also demonstrate a new technique of Raman enhancement driven by NRET in vdWhs. We achieve a 10-fold enhancement in the Raman intensity, enabling the observation of the otherwise invisible weak Raman modes. We establish the evidence for NRET-aided photoluminescence (PL) and Raman enhancement by modulating the degree of enhancement by systematically varying multiple parameters - donor material, acceptor material, their thickness, physical separation between donor and acceptor by insertion of spacer layer (hBN), sample temperature, and excitation wavelength. We also use the above parameters to decouple the effects of charge transfer and optical interference from NRET and establish a lower limit of the NRET-driven enhancement factor. We significantly modulate the strength of NRET by controlling the spectral overlap between 1L-TMD and SnSe2 through temperature variation. We show a remarkable agreement between such temperature-dependent Raman enhancement and the NRET-driven Raman polarizability model. We emphasize the advantages of using SnSe2 as a donor and elucidate the impact of various parameters on the PL enhancement using a rate equation framework. This NRET-driven enhancement can be used in tandem with other techniques and thus opens new avenues for improving quantum efficiency, coupling the advantages of uniform enhancement accessible across the entire junction area of vdWhs.

Further, we study the role of NRET in photocurrent generation across vdWhs by designing a vertical junction of SnSe2/multilayer-MoS2/TaSe2. We report the observation of an unusual negative differential photoconductance (NDPC) behaviour arising from the existence of NRET across the SnSe2/MoS2 junction. The modulation of NRET-driven NDPC characteristics with incident optical power results in a striking transition of the photocurrent's power law from sublinear to a superlinear regime. These observations highlight the nontrivial impact of NRET on the photoresponse of vdWhs and unfold possibilities to harness NRET in synergy with charge transfer.

The stacking angle between the individual layers in vdWhs provides another knob to tune their properties. The emergence of moiré patterns in twisted vdWhs creates superlattices where electronic bands fold into a series of minibands, inducing new phenomena. We experimentally demonstrate the PL emission from the moiré superlattice-induced intralayer exciton minibands in twisted TMD homobilayers using artificially stacked 1L-MoS2 layers at minimal twist angles. We also show the electrical tunability of these moiré excitons and the evolution of distinct moiré trions. We experimentally discern the localized versus delocalized nature of individual moiré peaks through different regimes of gating and optical excitation. Further, we discuss the gate-controlled valley coherence and resonant Raman scattering of moiré excitons. These experimental results provide unique insights into the moiré modulated optical properties of twisted bilayers.

Next, we focus on tuning the electrical characteristics of vdWhs to realize ambipolar injection, which is useful for LED and CMOS applications. vdW contacts offer atomically smooth and pristine interfaces without dangling bonds, coupled with a weak interaction at the interface. Such contacts help to achieve a completely de-pinned contact close to the Schottky-Mott limit. We demonstrate the weakly pinned nature of a vdW contact (TaSe2) by realizing improved ambipolar carrier injection into few-layer WS2 and WSe2 channels (compared to Au). Backward diodes offer a superior high-frequency response, temperature stability, radiation hardness, and 1/f noise performance than a conventional diode. We demonstrate a vdWh based backward diode by exploiting the giant staggered band offsets of the WSe2/SnSe2 junction. The diode exhibits an ultra-high reverse rectification ratio of ~2.1*10^4 up to a substantial bias of 1.5 V, with an excellent curvature coefficient of ~37 V^{-1}, outperforming existing backward diode reports. We efficiently modulate the carrier transport by varying the thickness of the WSe2 layer, the type of metal contacts employed, and the external gate and drain bias. We also show that the effective current transfer length at the vertical junction in vdWhs can be as large as the whole interface, which is in sharp contrast to the smaller transfer length (~100 nm) in typical metal-layered semiconductor junctions.

The results from this thesis widen the horizon for practical electronic, photonic, and optoelectronic applications of vdWhs.