Euyheon Hwang

Colloidal semiconductor nanocrystals (quantum dots) exhibit bright, narrowband, and spectrally tunable emission, making them attractive for photonic applications. Their emission properties are governed by closely spaced exciton fine structure, whose thermal mixing complicates control over key characteristics such as color purity, radiative rate, and polarization. Moreover, the lowest-energy excitonic state is typically non-emissive (dark), suppressing emission rates, particularly at cryogenic temperatures. Here we show that these limitations can be addressed by inducing controlled lattice distortion in zinc blende CdSe nanocrystals through pseudomorphic epitaxial growth of a ZnSe shell. This approach modifies the exciton fine structure by placing an emissive (bright) state at the lowest energy and increasing the separation between bright states derived from light- and heavy-hole subbands. These changes reduce thermal mixing, resulting in accelerated emission at low temperatures, sub-thermal linewidths, and polarization. These findings establish lattice distortion engineering as a strategy for controlling emission properties in colloidal nanocrystals.

Minimizing contact resistance between metal electrodes and perovskites remains a critical challenge in developing perovskite‐based electronic and optoelectronic devices. Despite extensive experimental and theoretical efforts to minimize the contact resistance, atomic‐level insights into metal‐perovskite interfaces remain insufficiently understood. In this paper, we investigate the electronic and transport properties of 2D hybrid organic–inorganic perovskites, specifically (BA) ₂ PbBr ₄ and (PEA) ₂ PbBr ₄ , which consist of monolayer bromide perovskite layers with different organic spacer molecules: n‐butylammonium (BA) and phenylethylammonium (PEA) cations. We provide a comprehensive analysis of the Au/perovskites interface, considering two contact configurations, i.e., top‐contact and edge‐contact. We perform first‐principles calculations and use the nonequilibrium Green's function formalism to calculate the transport characteristics. We find that a Schottky barrier forms in the top‐contact configuration, but the edge‐contact configuration shows ohmic behavior for both Au/(BA) ₂ PbBr ₄ and Au/(PEA) ₂ PbBr ₄ . The edge‐contact configuration also enables highly efficient charge injection with a negligible interfacial tunneling barrier due to Au‐induced midgap states. For the top contact configuration, Au/(PEA) ₂ PbBr ₄ shows better transport behavior compared to (BA) ₂ PbBr ₄ , which is attributed to its lower tunneling barrier and higher density of metal‐induced midgap states. This work provides theoretical guidelines for designing low‐resistance metal‐2D hybrid perovskite contacts.

We present a novel approach to electronic ferroelectricity by integrating monolayer graphene (MLG) with non-ferroelectric α-Fe ₂ O ₃ thin films. Unlike conventional ferroelectricity driven by ionic displacement, electronic ferroelectricity originates from charge dynamics. Electronic ferroelectricity has recently been observed in moiré systems, such as monolayer or few-layer graphene aligned with hexagonal boron nitride (hBN). However, ferroelectricity in these systems requires precise heterostructure alignment and low temperatures. Our proposed MLG/α-Fe ₂ O ₃ devices show robust electronic ferroelectricity without the need for moiré alignment and remain stable even at room temperature. The observed ferroelectric behavior in the proposed devices arises from electronic charge transfer between graphene and α-Fe ₂ O ₃ thin films, rather than lattice distortions. The polarization magnitude is continuously tunable with gate voltage, enabling multi-level non-volatile switching with high stability over repeated cycles. These findings provide MLG/Fe ₂ O ₃ heterostructures as a practical and stable platform for future electronic ferroelectric applications.

We investigate strain engineering in single-layer MoS2–Au heterostructures under biaxial and uniaxial tension applied along the zigzag and armchair directions. By systematically varying the strain conditions, we study how different strain configurations influence the electronic and interfacial properties of this two-dimensional (2D) material-based system. Under tensile strain, the Schottky barrier height (SBH) at the Au/MoS2 interface decreases and the interfacial binding energy increases, leading to a reduced van der Waals gap and enhanced electron tunneling probability. In contrast, compressive strain has the opposite effect, i.e., compressive strain increases the SBH and weakens the interface interaction. The SBH reduction under tensile strain gives rise to enhanced electron transfer from Au to MoS2, resulting in charge redistribution that effectively dopes MoS2 with electrons and shifts its Fermi level closer to the conduction band minimum. The tunability of SBH and tunneling barriers via strain highlights a viable strategy for optimizing metal–2D semiconductor contacts in nanoelectronics applications.

Efficient light generation from triplet states of organic molecules has been a hot yet demanding topic in academia and the display industry. Herein, we propose a strategy for developing triplet emitter by creating heterostructures of organic chromophores and transition metal dichalcogenides (TMDs). These heterostructures emit microsecond phosphorescence at room temperature, while their organic chromophores intrinsically exhibit millisecond phosphorescence under vibration dissipation-free conditions. This enhancement in phosphorescence is indicative of significantly enhanced spin-orbit coupling efficiency through coupling with TMDs. Through detailed studies on these hybrids from various perspectives, we elucidate key features of each component essential for generating microsecond triplet emission, including 2H-TMDs with heavy transition metals and aromatic carbonyl with an ortho-hydroxy group. Our intriguing findings open avenues for exploring the universal applicability of fast and stable hybrid triplet emitters.

Structural deformation modifies the bandgap, exciton fine structure and phonon energy of semiconductors, providing an additional knob to control their optical properties. The impact can be exploited in colloidal semiconductor quantum dots (QDs), wherein structural stresses can be imposed in three dimensions while defect formation is suppressed by controlling surface growth kinetics. Yet, the control over the structural deformation of QDs free from optically active defects has not been reached. Here, we demonstrate strain-graded CdSe-ZnSe core-shell QDs with compositionally abrupt interface by the coherent pseudomorphic heteroepitaxy. Resulting QDs tolerate mutual elastic deformation of varying magnitudes at the interface with high structural fidelity, allowing for spectrally stable and pure emission of photons at accelerated rates with near unity luminescence efficiency. We capitalize on the asymmetric strain effect together with the quantum confinement effect to expand emission envelope of QDs spanning the entire visible region and exemplify their use in photonic applications.

The in-plane anisotropy of electrical conductance in two-dimensional materials has garnered significant attention due to its potential in emerging device applications, offering an additional dimension to control carrier transport in 2D devices. However, previous research has primarily focused on the anisotropy within electrical channel, neglecting the significant impact of anisotropic electrical contacts of 2D materials. Here, we investigate anisotropic charge transport at the metal contacts of hBN-encapsulated ReS₂ using edge-contacted Field Effect Transistors. We observed the marked difference in contact resistance between the cross-b and b directions, suggesting that charge transport from the metal to ReS₂ is more efficient along the b direction. This difference in efficiency results in a substantial contact anisotropy, reaching ~70 at 77 K. Our findings indicate that the measured Schottky Barrier Height along the b direction is ~35 meV, which is smaller than along the cross-b direction. Moreover, the tunneling probability along the b direction is two times larger than along the cross-b direction. Our results indicate that both Schottky Barrier Height and tunneling amplitude are the primary contributors to the high contact anisotropy of ReS₂. This work provides a valuable guideline for understanding how in-plane orientation influences charge transport at metallic contacts in 2D devices.

Colloidal Ag(In,Ga)S₂ nanocrystals (AIGS NCs) with the band gap tunability by their size and composition within visible range have garnered surging interest. High absorption cross-section and narrow emission linewidth of AIGS NCs make them ideally suited to address the challenges of Cd-free NCs in wide-ranging photonic applications. However, AIGS NCs have shown relatively underwhelming photoluminescence quantum yield (PL QY) to date, primarily because coherent heteroepitaxy has not been realized. Here, we report the heteroepitaxy for AIGS-AgGaS₂ (AIGS-AGS) core-shell NCs bearing near-unity PL QYs in almost full visible range (460 to 620 nm) and enhanced photochemical stability. Key to the successful growth of AIGS-AGS NCs is the use of the Ag-S-Ga(OA)₂ complex, which complements the reactivities among cations for both homogeneous AIGS cores in various compositions and uniform AGS shell growth. The heteroepitaxy between AIGS and AGS results in the Type I heterojunction that effectively confines charge carriers within the emissive core without optically active interfacial defects. AIGS-AGS NCs show higher extinction coefficient and narrower spectral linewidth compared to state-of-the-art heavy metal-free NCs, prompting their immediate use in practicable applications including displays and luminescent solar concentrators (LSCs).

Carrier multiplication via impact ionization in two-dimensional (2D) layered materials is a very promising process for manufacturing high-performance devices because the multiplication has been reported to overcome thermodynamic conversion limits. Given that 2D layered materials exhibit highly anisotropic transport properties, understanding the directionally-dependent multiplication process is necessary for device applications. In this study, the anisotropy of carrier multiplication in the 2D layered material, WSe₂, is investigated. To study the multiplication anisotropy of WSe₂, both lateral and vertical WSe₂ field effect transistors (FETs) are fabricated and their electrical and transport properties are investigated. We find that the multiplication anisotropy is much bigger than the transport anisotropy, i.e., the critical electric field (E_CR) for impact ionization of vertical WSe₂ FETs is approximately ten times higher than that of lateral FETs. To understand the experimental results we calculate the average energy of the carriers in the proposed devices under strong electric fields by using the Monte Carlo simulation method. The calculated average energy is strongly dependent on the transport directions and we find that the critical electric field for impact ionization in vertical devices is approximately one order of magnitude larger than that of the lateral devices, consistent with experimental results. Our findings provide new strategies for the future development of low-power electric and photoelectric devices. Graphical Abstract