The ability to directly observe electronic band structure in modern nanoscale field-effect devices could transform understanding of their physics and function. One could, for example, visualize local changes in the electrical and chemical potentials as a gate voltage is applied. One could also study intriguing physical phenomena such as electrically induced topological transitions and many-body spectral reconstructions. Here we show that submicron angle-resolved photoemission (-ARPES) applied to two-dimensional (2D) van der Waals heterostructures affords this ability. In graphene devices, we observe a shift of the chemical potential by 0.6 eV across the Dirac point as a gate voltage is applied. In several 2D semiconductors we see the conduction band edge appear as electrons accumulate, establishing its energy and momentum, and observe significant band-gap renormalization at low densities. We also show that -ARPES and optical spectroscopy can be applied to a single device, allowing rigorous study of the relationship between gate-controlled electronic and excitonic properties.Angle resolved photoemission spectroscopy (ARPES), in which the energy and momentum of photoemitted electrons are measured from a sample subjected to a spectrally narrow ultraviolet or X-ray excitation, is a powerful technique that yields the momentum-dependent single-electron band structure and chemical potential in a solid with essentially no assumptions. It probes only electron states near the surface, and so cannot be applied to conventional semiconductor devices. It is, however, very effective when applied to 2D materials and has been used extensively to study the bands in graphene 1 , monolayer transition metal dichalcogenides 2-7 , and others 8,9 . Furthermore, µ-ARPES (with a micron-scale beam spot) can be performed 10 on 2D heterostructures (2DHSs) 11 made of stacked exfoliated 2D materials 12-14 , suggesting the possibility of monitoring electronic structure during actual device operation. We demonstrate here that momentum-resolved electronic spectra can indeed be obtained during reversible electrostatic gating, enabling direct visualization of chemical potential shifts and band structure changes controlled by the gate electric field.A limitation of ARPES is that it probes only occupied electron states, and so a semiconductor must first be electron-doped in order to obtain a signal from the conduction band. The usual approach is to deposit alkali metal atoms 1-7,15 which act as an n-type dopant, but this has several limitations: the density cannot be controlled accurately; it can only be reversed by high-temperature annealing; it introduces disorder through the random positions of the dopants; and it chemically perturbs the electronic structure in ways that are hard to calculate. Electrostatic doping has none of these disadvantages, and the accessible carrier densities are most relevant to practical devices.We first validate our technique using graphene, and then go on to apply it to the 2D transition metal dichalcogenide (TMD) sem...

HIV-1 entry requires the cell surface expression of CD4 and either the CCR5 or CXCR4 coreceptors on host cells. Individuals homozygous for the ccr5Δ32 polymorphism do not express CCR5 and are protected from infection by CCR5-tropic (R5) virus strains. As an approach to inactivating CCR5, we introduced CCR5-specific zinc-finger nucleases into human CD4+ T cells prior to adoptive transfer, but the need to protect cells from virus strains that use CXCR4 (X4) in place of or in addition to CCR5 (R5X4) remains. Here we describe engineering a pair of zinc finger nucleases that, when introduced into human T cells, efficiently disrupt cxcr4 by cleavage and error-prone non-homologous DNA end-joining. The resulting cells proliferated normally and were resistant to infection by X4-tropic HIV-1 strains. CXCR4 could also be inactivated in ccr5Δ32 CD4+ T cells, and we show that such cells were resistant to all strains of HIV-1 tested. Loss of CXCR4 also provided protection from X4 HIV-1 in a humanized mouse model, though this protection was lost over time due to the emergence of R5-tropic viral mutants. These data suggest that CXCR4-specific ZFNs may prove useful in establishing resistance to CXCR4-tropic HIV for autologous transplant in HIV-infected individuals.

Real-space imaging reveals rich microscopic details of the quantum spin Hall edge conduction in monolayer WTe2.

In electronic and optoelectronic devices made from van der Waals heterostructures, electric fields can induce substantial band structure changes which are crucial to device operation but cannot usually be directly measured. Here, we use spatially resolved angle-resolved photoemission spectroscopy to monitor changes in band alignment of the component layers, corresponding to band structure changes of the composite heterostructure system, that are produced by electrostatic gating. Our devices comprise graphene on a monolayer semiconductor, WSe2 or MoSe2, atop a boron nitride dielectric and a graphite gate. Applying a gate voltage creates an electric field that shifts the semiconductor bands relative to those in the graphene by up to 0.2 eV. The results can be understood in simple terms by assuming that the materials do not hybridize.