At the turn of this century, Herbert Kroemer, the 2000 Nobel Prize winner in Physics, famously commented that "the interface is the device". This statement has since opened up unparalleled opportunities at the interface of conventional three-dimensional (3D) materials (H. Kroemer, Quasi-Electric and Quasi-Magnetic Fields in Non-Uniform Semiconductors, RCA Rev., 1957, 18, 332-342). More than a decade later, Sir Andre Geim and Irina Grigorieva presented their views on 2D heterojunctions which further cultivated broad interests in the 2D materials field. Currently, advances in two-dimensional (2D) materials enable us to deposit layered materials that are only one or few unit-cells in thickness to construct sharp in-plane and out-of-plane interfaces between dissimilar materials, and to be able to fabricate novel devices using these cutting-edge techniques. The interface alone, which traditionally dominated overall device performance, thus has now become the device itself. Fueled by recent progress in atomically thin materials, we are now at the ultimate limit of interface physics, which brings to us new and exciting opportunities, with equally demanding challenges. This paper endeavors to provide stalwarts and newcomers a perspective on recent advances in synthesis, fundamentals, applications, and future prospects of a large variety of heterojunctions of atomically thin materials.

a Zirconium trisulphide (ZrS 3 ), a member of the layered transition metal trichalcogenides (TMTCs) family, has been studied by angle-resolved photoluminescence spectroscopy (ARPLS). The synthesized ZrS 3 layers possess a pseudo one-dimensional nature where each layer consists of ZrS 3 chains extending along the b-lattice direction. Our results show that the optical properties of few-layered ZrS 3 are highly anisotropic as evidenced by large PL intensity variation with the polarization direction. Light is efficiently absorbed when the E-field is polarized along the chain (b-axis), but the field is greatly attenuated and absorption is reduced when it is polarized vertical to the 1D-like chains as the wavelength of the exciting light is much longer than the width of each 1D chain. The observed PL variation with polarization is similar to that of conventional 1D materials, i.e., nanowires, and nanotubes, except for the fact that here the 1D chains interact with each other giving rise to a unique linear dichroism response that falls between the 2D ( planar) and 1D (chain) limit. These results not only mark the very first demonstration of PL polarization anisotropy in 2D systems, but also provide novel insight into how the interaction between adjacent 1D-like chains and the 2D nature of each layer influences the overall optical anisotropy of pseudo-1D materials. Results are anticipated to have an impact on optical technologies such as polarized detectors, near-field imaging, communication systems, and bio-applications relying on the generation and detection of polarized light.

Modulation of weak interlayer interactions between quasi-two-dimensional atomic planes in the transition metal dichalcogenides (TMDCs) provides avenues for tuning their functional properties. Here we show that above-gap optical excitation in the TMDCs leads to an unexpected large-amplitude, ultrafast compressive force between the two-dimensional layers, as probed by in situ measurements of the atomic layer spacing at femtosecond time resolution. We show that this compressive response arises from a dynamic modulation of the interlayer van der Waals interaction and that this represents the dominant light-induced stress at low excitation densities. A simple analytic model predicts the magnitude and carrier density dependence of the measured strains. This work establishes a new method for dynamic, nonequilibrium tuning of correlation-driven dispersive interactions and of the optomechanical functionality of TMDC quasi-two-dimensional materials.

Transition metal trichalcogenides form a class of layered materials with strong in-plane anisotropy. For example, titanium trisulfide (TiS3) whiskers are made out of weakly interacting TiS3 layers, where each layer is made of weakly interacting quasi-one-dimensional chains extending along the b axis. Here we establish the unusual vibrational properties of TiS3 both experimentally and theoretically. Unlike other two-dimensional systems, the Raman active peaks of TiS3 have only out-of-plane vibrational modes, and interestingly some of these vibrations involve unique rigid-chain vibrations and S–S molecular oscillations. High-pressure Raman studies further reveal that the AgS–S S-S molecular mode has an unconventional negative pressure dependence, whereas other peaks stiffen as anticipated. Various vibrational modes are doubly degenerate at ambient pressure, but the degeneracy is lifted at high pressures. These results establish the unusual vibrational properties of TiS3 with strong in-plane anisotropy, and may have relevance to understanding of vibrational properties in other anisotropic two-dimensional material systems.

Monolayered MoSe2 is a promising new material to investigate advanced light-matter coupling as it hosts stable and robust excitons with comparably narrow optical resonances. In this work, we investigate the evolution of the lowest lying excitonic transition, the so-called A-valley exciton, with temperature. We find a strong, phonon-induced temperature broadening of the resonance, and more importantly, a reduction of the oscillator strength for increased temperatures. Based on these experimentally extracted, temperature dependent parameters, we apply a coupled oscillator model to elucidate the possibility to observe the strong coupling regime between the A-exciton and a microcavity resonance in three prototypical photonic architectures with varying mode volumes. We find that the formation of exciton-polaritons up to ambient conditions in short, monolithic dielectric and Tamm-based structures seems feasible. In contrast, a temperature-induced transition into the weak coupling regime can be expected for structures with extended effective cavity length. Based on these findings, we calculate and draw the phase diagram of polariton Bosonic condensation in a MoSe2 cavity.

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Cadmium sulfide (CdS) nanostructures have attracted a significant amount of attention for a variety of optoelectronic applications including photovoltaic cells, semiconductor lasers, and solid-state laser refrigeration due to their direct bandgap around 2.42 eV and high radiative quantum efficiency. Nanoribbons (NRs) of CdS have been claimed to laser cool following excitation at 514 and 532 nm wavelengths by the annihilation of optical phonons during anti-Stokes photoluminescence. To explore this claim, we demonstrate a novel optomechanical experimental technique for microthermometry of a CdSNR cantilever using Young’s modulus as the primary temperature-dependent observable. Measurements of the cantilever’s fundamental acoustic eigenfrequency at low laser powers showed a red-shift in the eigenfrequency with increasing power, suggesting net heating. At high laser powers, a decrease in the rate of red-shift of the eigenfrequency is explained using Euler–Bernoulli elastic beam theory, considering Hookean optical-trapping force. A predicted imaginary refractive index for CdSNR based on experimental temperature measurement agrees well with a heat transfer analysis that predicts the temperature distribution within the cantilever and the time required to reach steady state (<100 μs). This approach is useful for investigating solid-state laser refrigeration of a wide variety of material systems without the need for complex pump/probe spectroscopy.

Atomically thin quasi-2D GaSe flakes are synthesized via van der Waals (vdW) epitaxy on a polar Si (111) surface. The bandgap is continuously tuned from its commonly accepted value at 620 down to the 700 nm range, only attained previously by alloying Te into GaSe (GaSex Te1- x ). This is accomplished by manipulating various vdW epitaxy kinetic factors, which allows the choice bet ween screw-dislocation-driven and layer-bylayer growth, and the design of different morphologies with different material-substrate interaction (strain) energies.