Grant Aivazian

Electrical control of neutral and charged excitons in a monolayer semiconductor

Monolayer group-VI transition metal dichalcogenides have recently emerged as semiconducting alternatives to graphene in which the true two-dimensionality is expected to illuminate new semiconducting physics. Here we investigate excitons and trions (their singly charged counterparts), which have thus far been challenging to generate and control in the ultimate two-dimensional limit. Utilizing high-quality monolayer molybdenum diselenide, we report the unambiguous observation and electrostatic tunability of charging effects in positively charged (X(+)), neutral (X(o)) and negatively charged (X(-)) excitons in field-effect transistors via photoluminescence. The trion charging energy is large (30 meV), enhanced by strong confinement and heavy effective masses, whereas the linewidth is narrow (5 meV) at temperatures <55 K. This is greater spectral contrast than in any known quasi-two-dimensional system. We also find the charging energies for X(+) and X(-) to be nearly identical implying the same effective mass for electrons and holes.

Observation of Long-Lived Interlayer Excitons in Monolayer MoSe2-WSe2 Heterostructures

Van der Waals bound heterostructures constructed with two-dimensional materials, such as graphene, boron nitride and transition metal dichalcogenides, have sparked wide interest in device physics and technologies at the two-dimensional limit. One highly coveted heterostructure is that of differing monolayer transition metal dichalcogenides with type-II band alignment, with bound electrons and holes localized in individual monolayers, that is, interlayer excitons. Here, we report the observation of interlayer excitons in monolayer MoSe2-WSe2 heterostructures by photoluminescence and photoluminescence excitation spectroscopy. We find that their energy and luminescence intensity are highly tunable by an applied vertical gate voltage. Moreover, we measure an interlayer exciton lifetime of ~1.8 ns, an order of magnitude longer than intralayer excitons in monolayers. Our work demonstrates optical pumping of interlayer electric polarization, which may provoke further exploration of interlayer exciton condensation, as well as new applications in two-dimensional lasers, light-emitting diodes and photovoltaic devices.

Electrical tuning of valley magnetic moment through symmetry control in bilayer MoS2

Electric fields can break the structural inversion symmetry in bilayer 2D materials, providing a way of tuning the magnetic moment and Berry curvature. This effect can be probed directly in bilayer MoS2 using optical measurements.

Vapor–Solid Growth of High Optical Quality MoS2 Monolayers with Near-Unity Valley Polarization

Monolayers of transition metal dichalcogenides (TMDCs) are atomically thin direct-gap semiconductors with potential applications in nanoelectronics, optoelectronics, and electrochemical sensing. Recent theoretical and experimental efforts suggest that they are ideal systems for exploiting the valley degrees of freedom of Bloch electrons. For example, Dirac valley polarization has been demonstrated in mechanically exfoliated monolayer MoS2 samples by polarization-resolved photoluminescence, although polarization has rarely been seen at room temperature. Here we report a new method for synthesizing high optical quality monolayer MoS2 single crystals up to 25 μm in size on a variety of standard insulating substrates (SiO2, sapphire, and glass) using a catalyst-free vapor-solid growth mechanism. The technique is simple and reliable, and the optical quality of the crystals is extremely high, as demonstrated by the fact that the valley polarization approaches unity at 30 K and persists at 35% even at room temperature, suggesting a virtual absence of defects. This will allow greatly improved optoelectronic TMDC monolayer devices to be fabricated and studied routinely.

Magnetic control of valley pseudospin in monolayer WSe2

Supplementary Text S1. Computing the valley Zeeman effect in multiple samples. The peak splitting due to the valley Zeeman effect is small compared to the width of the photoluminescence (PL) peaks so care must be taken in determining the Zeeman splitting. The peaks are slightly asymmetric, with shape varying somewhat with magnetic field, and do not conform well to a Gaussian or Lorentzian peak shape. We use two methods to determine the peak position and hence the Zeeman splitting, both of which make no assumptions about the peak shape. As shown in Figure 1c, they agree very well. The first, “max point”, simply finds the 15 points in each spectrum with the most counts and assigns the peak position to the median value of these points. This method is insensitive to the trion peak, which is too far away to influence these points; however, it is more sensitive to noise as it only considers a few points. The second method, “weighted average”, computes the “center of mass” of the peak, , where is the PL spectral density and E is photon energy. In this method the effect of noise is greatly reduced because it makes use of all the several hundred points that make up the spectrum, but on the other hand it is more sensitive to the trion peak, which will tend to over-weight the low-energy side of the peak. However, since the valley exciton Zeeman splitting is small and we are interested in the difference between the σ+ and σpeaks, the weak trion effects on both peaks tend to balance each other out. The data in Fig. 1c and Fig. S1a are from two different samples. We can see that the splitting as a function of magnetic field obtained by these two different methods has little difference. Eight samples were measured and all were observed to have a splitting linear in the applied field. In Figure S1b we plot the fitted slope of the splittings from all the samples, in units of Bohr magnetons. The data presented in the main text is from the fifth sample. We see that the data can be split into two groups with mean values of 1.57 μB and 2.86 μB. The origin of this bimodal distribution in the g-factors is unclear due to the lack of understanding of what external factors can affect the g-factors in these new materials. Future studies will be necessary to quantitatively determine the effect of variables such as strain, doping, and substrate on the magnetic properties. However, all samples show similar behavior in their valley polarization as a function of the applied field (i.e., the “X” and “V” patterns).

Probing transconductance spatial variations in graphene nanoribbon field-effect transistors using scanning gate microscopy

We have used scanning gate microscopy to probe local transconductance in graphene nanoribbon (GNR) field-effect transistors (FETs) fabricated from chemical vapor deposition-grown graphene. Particularly, nanometer-scale (≤100 nm, resolution limited) areas characterized by significant transconductance spatial variations were observed along the FET channel. These were attributed to the impurities at or close to the edges of the GNRs. Our results further show that a single such impurity site in a long-channel (∼2 μm) GNR FET can essentially control the global device characteristics. This finding demonstrates the importance of controlling the spatial inhomogeneity of electronic properties in graphene and related nanostructures in order to realize their envisioned applications in new electronics.

Multiple hot-carrier collection in photo-excited graphene Moiré superlattices

Morié-engineered graphene devices can collect multiple electrons per absorbed photon, promising efficient optoelectronics. In conventional light-harvesting devices, the absorption of a single photon only excites one electron, which sets the standard limit of power-conversion efficiency, such as the Shockley-Queisser limit. In principle, generating and harnessing multiple carriers per absorbed photon can improve efficiency and possibly overcome this limit. We report the observation of multiple hot-carrier collection in graphene/boron-nitride Moiré superlattice structures. A record-high zero-bias photoresponsivity of 0.3 A/W (equivalently, an external quantum efficiency exceeding 50%) is achieved using graphene’s photo-Nernst effect, which demonstrates a collection of at least five carriers per absorbed photon. We reveal that this effect arises from the enhanced Nernst coefficient through Lifshtiz transition at low-energy Van Hove singularities, which is an emergent phenomenon due to the formation of Moiré minibands. Our observation points to a new means for extremely efficient and flexible optoelectronics based on van der Waals heterostructures.

Many-body effects in nonlinear optical responses of 2D layered semiconductors

We performed ultrafast degenerate pump-probe spectroscopy on monolayer WSe2 near its exciton resonance. The observed differential reflectance signals exhibit signatures of strong many-body interactions including the exciton-exciton interaction and free carrier induced band gap renormalization. The exciton-exciton interaction results in a resonance blue shift which lasts for the exciton lifetime (several ps), while the band gap renormalization manifests as a resonance red shift with several tens ps lifetime. Our model based on the many-body interactions for the nonlinear optical susceptibility fits well the experimental observations. The power dependence of the spectra shows that with the increase of pump power, the exciton population increases linearly and then saturates, while the free carrier density increases superlinearly, implying that exciton Auger recombination could be the origin of these free carriers. Our model demonstrates a simple but efficient method for quantitatively analyzing the spectra, and indicates the important role of Coulomb interactions in nonlinear optical responses of such 2D materials.

Photo-Nernst current in graphene

When laser light is focused onto graphene devices in a magnetic field a long-range photo-Nernst effect causes photocurrents to be generated along the free edges.