Chemical methods developed over the past two decades enable preparation of colloidal nanocrystals with uniform size and shape. These Brownian objects readily order into superlattices. Recently, the range of accessible inorganic cores and tunable surface chemistries dramatically increased, expanding the set of nanocrystal arrangements experimentally attainable. In this review, we discuss efforts to create next-generation materials via bottom-up organization of nanocrystals with preprogrammed functionality and self-assembly instructions. This process is often driven by both interparticle interactions and the influence of the assembly environment. The introduction provides the reader with a practical overview of nanocrystal synthesis, self-assembly, and superlattice characterization. We then summarize the theory of nanocrystal interactions and examine fundamental principles governing nanocrystal self-assembly from hard and soft particle perspectives borrowed from the comparatively established fields of micrometer colloids and block copolymer assembly. We outline the extensive catalog of superlattices prepared to date using hydrocarbon-capped nanocrystals with spherical, polyhedral, rod, plate, and branched inorganic core shapes, as well as those obtained by mixing combinations thereof. We also provide an overview of structural defects in nanocrystal superlattices. We then explore the unique possibilities offered by leveraging nontraditional surface chemistries and assembly environments to control superlattice structure and produce nonbulk assemblies. We end with a discussion of the unique optical, magnetic, electronic, and catalytic properties of ordered nanocrystal superlattices, and the coming advances required to make use of this new class of solids.

Predicting structure from the attributes of a material's building blocks remains a challenge and central goal for materials science. Isolating the role of building block shape for self-assembly provides insight into the ordering of molecules and the crystallization of colloids, nanoparticles, proteins, and viruses. We investigated 145 convex polyhedra whose assembly arises solely from their anisotropic shape. Our results demonstrate a remarkably high propensity for thermodynamic self-assembly and structural diversity. We show that from simple measures of particle shape and local order in the fluid, the assembly of a given shape into a liquid crystal, plastic crystal, or crystal can be predicted.

We detect electroluminescence in single layer molybdenum disulphide (MoS2) field-effect transistors built on transparent glass substrates. By comparing absorption, photoluminescence, and electroluminescence of the same MoS2 layer, we find that they all involve the same excited state at 1.8eV. The electroluminescence has pronounced threshold behavior and is localized at the contacts. The results show that single layer MoS2, a direct band gap semiconductor, is promising for novel optoelectronic devices, such as 2-dimensional light detectors and emitters. Here, we report electrically excited luminescence in 1L-MoS 2 FETs, and study the underlying emission mechanism. We find that the electroluminescence occurs via hot carriers and is localized in the contacts region. The observed photoluminescence and electroluminescence arise from the same excited state at 1.8eV. Molybdenum disulphide (MoS1L-MoS 2 crystals are produced by micromechanical cleavage of bulk MoS 2 (Structure Probe Inc.-SPI, Natural Molybdenite) on 100nm SiO 2 . As for the case of graphene [20], interference allows visibility and counting the number of layers, Fig.1a. Due to the different dielectric properties, an optimum thickness of 100nm SiO 2 is well suited for MoS 2 [21]. The presence of monolayers is then confirmed by performing PL measurements, Fig.1b. The PL spectrum of 1L-MoS 2 exhibits two bands at 2eV and 1.8eV (Fig.1b) associated with excitonic transitions at the K point of the Brillouin zone[4]. The energy difference of 0.2eV has been attributed to the degeneracy breaking of the valence band due to spin-orbit coupling [4,7,8,22]. As compared to bulk MoS 2 , Fig.1b, 1L-MoS 2 does not have a peak at 1.4eV [3,4], associated with the indirect band gap [12]. In addition 1L-MoS 2 exhibits a stronger PL intensity compared to bulk MoS 2 [3, 4] due to the direct band gap. Another evidence for 1L-MoS 2 comes from the analysis of the Raman spectrum, Fig.1d. The peak at∼385cm −1 corresponds to the in plane (E 1 2g ) mode [23], while that at ∼404 cm −1 is attributed to the out of plane (A 1g ) mode [23]. The E 1 2g mode softens and A 1g mode stiffens with increasing layer thickness[23], similar to what happens for other layered materials, where the bond distance changes with number of layers [24]. The frequency difference between these two modes can be used as a signature of 1L-MoS 2 [23].1L-MoS 2 flakes are then transferred onto glass substrates by using a poly(methyl methacrylate) (PMMA) based transfer technique, similar to that previously used to transfer graphene onto optical fibre cores [25]. This process involves spin coating two layers of 495K PMMA and one layer of 950K PMMA on the substrate where flakes are deposited. The samples are subsequently immersed in de-ionized (DI) water at 90 • C for 1h, resulting in the detachment of the polymer film, due to the intercalation of water at the polymer-SiO 2 interface. MoS 2 flakes stick to the PMMA, and can thus be removed from the original substrate and mechanically transferred onto glass substrates [2...

Black phosphorus is a layered semiconductor that is intensely researched in viewof applications in optoelectronics. In this Letter, we investigate a multi-layer black phosphorus photo-detector that is capable of acquiring high-contrast (V>0.9) images both in the visible (λ VIS =532nm) as well as in the infrared (λ IR =1550nm) spectral regime. In a first step, by using photocurrent microscopy, we map the active area of the device and we characterize responsivity and gain.In a second step, by deploying the black phosphorus device as a point-like detector in a confocal microsope setup, we acquire diffraction-limited optical images with sub-micron resolution. The results demonstrate the usefulness of black phosphorus as an optoelectronic material for hyperspectral imaging applications.