AbstractPhotoluminescence (PL), a photo-excited spontaneous emission process, provides a wealth of optical and electronic properties of materials, which enable microscopic and spectroscopic imaging, biomedical sensing and diagnosis, and a range of photonic device applications. However, conventional far-field PL measurements have limitations in sensitivity and spatial resolution, especially to investigate single nano-materials or nano-scale dimension of them. In contrast, tip-enhanced photoluminescence (TEPL) nano-spectroscopy provides an extremely high sensitivity with <10 nm spatial resolution, which allows the desired nano-scale characterizations. With outstanding and unique optical properties, low-dimensional quantum materials have recently attracted much attention, and TEPL characterizations, i. e., probing and imaging, and even control at the nano-scale, have been extensively studied. In this review, we discuss the fundamental working mechanism of PL enhancement by plasmonic tip, and then highlight recent advances in TEPL studies for low-dimensional quantum materials. Finally, we discuss several remaining challenges of TEPL nano-spectroscopy and nano-imaging, such as implementation in non-ambient media and in situ environments, limitations in sample structure, and control of near-field polarization, with perspectives of the approach and its applications.

The tunability of the bandgap, absorption and emission energies, photoluminescence (PL) quantum yield, exciton transport, and energy transfer in transition metal dichalcogenide (TMD) monolayers provides a new class of functions for a wide range of ultrathin photonic devices. Recent strain‐engineering approaches have enabled to tune some of these properties, yet dynamic control at the nanoscale with real‐time and ‐space characterizations remains a challenge. Here, a dynamic nano‐mechanical strain‐engineering of naturally‐formed wrinkles in a WSe2 monolayer, with real‐time investigation of nano‐spectroscopic properties is demonstrated using hyperspectral adaptive tip‐enhanced PL (a‐TEPL) spectroscopy. First, nanoscale wrinkles are characterized through hyperspectral a‐TEPL nano‐imaging with <15 nm spatial resolution, which reveals the modified nano‐excitonic properties by the induced tensile strain at the wrinkle apex, for example, an increase in the quantum yield due to the exciton funneling, decrease in PL energy up to ≈10 meV, and a symmetry change in the TEPL spectra caused by the reconfigured electronic bandstructure. Then the local strain is dynamically engineered by pressing and releasing the wrinkle apex through an atomic force tip control. This nano‐mechanical strain‐engineering allows to tune the exciton dynamics and emission properties at the nanoscale in a reversible fashion. In addition, a systematic switching and modulation platform of the wrinkle emission is demonstrated, which provides a new strategy for robust, tunable, and ultracompact nano‐optical sources in atomically thin semiconductors.

The activation of plastic deformation mechanisms determines the mechanical behavior of crystalline materials. However, the complexity of plastic deformation and the lack of a unified theory of plasticity have seriously limited the exploration of the full capacity of metals. Current efforts to design high-strength structural materials in terms of stacking fault energy have not significantly reduced the laborious trial and error works on basic deformation properties. To remedy this situation, here we put forward a comprehensive and transparent theory for plastic deformation of face-centered cubic metals. This is based on a microscopic analysis that, without ambiguity, reveals the various deformation phenomena and elucidates the physical fundaments of the currently used phenomenological correlations. We identify an easily accessible single parameter derived from the intrinsic energy barriers, which fully specifies the potential diversity of metals. Based entirely on this parameter, a simple deformation mode diagram is shown to delineate a series of convenient design criteria, which clarifies a wide area of material functionality by texture control.planar fault | twinning | slip | molecular dynamics E lastic deformation of metals is fully described by Hooke's law, connecting stress and strain via the usual elastic parameters. In contrast, plasticity is a property that originates from dislocations and involves transitions over various competing energy barriers. A phenomenological description of the plastic regime based merely on the stacking fault energy (SFE) has been widely used by material scientists (1-11), even though it is not at all clear whether such an equilibrium property can really capture the complexity of plastic deformation. To try to mend this situation, researchers have introduced the so-called generalized planar fault (GPF) energy, which comprises several intrinsic energy barriers (IEBs) and thus provides detailed information on the deformation process itself (12, 13). Nevertheless, the GPF energy has not yet been fully recognized because of its inherent difficulty in experimental validation (14). In the present paper we solve this and introduce an approach that is fully transparent, where there are no such ambiguities.Face-centered cubic (fcc) metals possess three distinct deformation mechanisms: stacking fault (SF), twinning (TW), and full slip (SL) (2,(5)(6)(7)15). Many studies tried to relate the deformation of fcc metals to the GPF energy. For example, the competition between TW and SL was explained by a relative change between the intrinsic energy barriers, assuming the activation of the lowest barrier mode (16)(17)(18). However, such a simplified picture was found to be inadequate to elucidate the simultaneous activation of different modes seen in experiments (2,5,6,15). It has emerged that additional factors should be taken into account in conjunction with the GPF energy for describing the deformation behavior.The microstructure of materials, such as grain size and orientation, is known to in...

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