All-inorganic perovskites nanostructures, such as CsPbCl 3 nanocrystals (NCs), are promising in many applications including light-emitting diodes, photovoltaics, and photodetectors. Despite the impressive performance that was demonstrated, a critical issue remains due to the instability of the perovskites in ambient. Herein, we report a method of passivating crystalline CsPbCl 3 NC surfaces with 3-mercaptopropionic acid (MPA), and superior ambient stability is achieved. The printing of these colloidal NCs on the channel of graphene field-effect transistors (GFETs) on solid Si/SiO 2 and flexible polyethylene terephthalate substrates was carried out to obtain CsPbCl 3 NCs/GFET heterojunction photodetectors for flexible and visible-blind ultraviolet detection at wavelength below 400 nm. Besides ambient stability, the additional benefits of passivating surface charge trapping by the defects on CsPbCl 3 NCs and facilitating highefficiency charge transfer between the CsPbCl 3 NCs and graphene were provided by MPA. Extraordinary optoelectronic performance was obtained on the CsPbCl 3 NCs/graphene devices including a high ultraviolet responsivity exceeding 10 6 A/W, a high detectivity of 2 × 10 13 Jones, a fast photoresponse time of 0.3 s, and ambient stability with less than 10% degradation of photoresponse after 2400 h. This result demonstrates the crucial importance of the perovskite NC surface passivation not only to the performance but also to the stability of the perovskite optoelectronic devices.

electromagnetic enhancement (EM) and chemical enhancement (CM). The EM involves the local electromagnetic field enhancement that is typically attributed to the localized surface plasmonic resonance (LSPR) of free charge carriers at the surface of the metal nanostructures induced by the incident light. The LSPR wavelength is determined primarily by the free charge carrier concentration of the metal with a minor effect of the dimension and shape of the metal nanostructures. Molecules positioned [2] close to the LSPR nanostructures experience an enhanced evanescent electromagnetic field as compared to the incident excitation. This EM enhancement directly depends on the morphology of the metal surface, the wavelength of the incident light, and the dielectric constant of the surrounding medium of the metal. The EM enhancement factor can reach over 10 8 to enable ultrasensitive SERS detection down to the single-molecule level. [4][5][6] The CM is induced by the charge transfer between the SERS substrate and molecule with an enhancement factor typically on the order of 10 1 to 10 3 . [7][8][9] The CM effect is dictated by the interface electronic structures between the analyte and substrate and can be optimized by selecting a substrate with favorable band alignment with the highestoccupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO) at the interface where the analyte (or probe molecule) bond to the substrate. Thus, tuning of the substrate electronic structure is important to an enhanced CM effect. [10] This has prompted intensive research exploring graphene-based SERS substrates considering the unique 2D atomically flat surface with delocalized π bonds, chemical inertness, biological compatibility, superior electronic and photonic properties, and the intrinsic Fermi energy at ≈4.5 eV that is compatible, as well as tunable, for CM enhancement with a large number of probe molecules. [7,8,11,12] Therefore, graphene is an excellent SERS substrate primarily due to the CM effect with the adsorbed molecules and the enhancement factor is quantitatively affected by the alignment of the probe molecule electronic structure with the Fermi level of graphene. [3,8] The EM and CM enhancement factors may be combined by adding metal nanostructures on graphene. [7,13] Since the Two-dimensional transition metal dichalcogenides (TMDs)/graphene van der Waals (vdW) heterostructures integrate the superior light-solid interaction in TMDs and charge mobility in graphene, and therefore are promising for surface-enhanced Raman spectroscopy (SERS). Herein, a novel TMD (MoS 2 and WS 2 ) nanodome/graphene vdW heterostructure SERS substrate, on which an extraordinary SERS sensitivity is achieved, is reported. Using fluorescent Rhodamine 6G (R6G) as probe molecules, the SERS sensitivity is in the range of 10 −11 to 10 −12 m on the TMD nanodomes/ graphene vdW heterostructure substrates using 532 nm Raman excitation, which is comparable to the best sensitivity reported so far using plasmonic metal nanostructures/graphene ...

In this study, we report a comprehensive assessment on the elastic and electronic properties of 792 possible MAX (M nþ1 AX n ) phases with n ¼ 1-4 using ab initio methods. These crystals are then screened based on their elastic and thermodynamic stability resulting in a large database of 665 viable crystals. All the experimentally verified MAX phases passed the screening. Various correlations among and between them are fully explored. In particular, the key elements in the interdependence between the elastic properties together with mechanical parameter derived from them and the electronic structure are identified. Detailed analysis of various correlation plots shows that there is a clear correspondence between bulk modulus K and total bond order density (TBOD). Calculations show a marked difference between the carbides and nitrides. This database is also used to test the efficacy of data mining algorithms for materials genome. We further identified several thermodynamically stable new MAX phases with unusual mechanical parameters that have never been synthesized in the laboratory or theoretically investigated. The complete database on the elastic and electronic structure together with the mechanical parameters for these 665 MAX phases compounds are included in the Supplementary Materials and fully accessible.

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