News & Comment

By mass, iron is the most common element on earth and the most widely utilized metal in industry. Over 30 years of research have shown that iron is a poor choice for practical applications in solar energy conversion. Hematite and other ferric oxide polymorphs are competent for water oxidation in photoelectrochemical cells, but just barely. Photovoltaics based on iron oxide or sulfide materials are inefficient and the inclusion of even trace iron in silicon significantly lowers the efficiency of today’s commercial photovoltaics. In dye-sensitized solar cells (DSSCs), inclusion of an iron center in a dye molecule results in devices with poor efficiencies that are of no practical value.¹ Iron-based materials and compounds function for light-driven electron transfer and catalysis, but just well enough to give some hope to highly optimistic scientists in academic labs. In the final analysis, iron consistently continues to disappoint and one can safely conclude that it is best to keep iron out of solar cells. Until now Harlang et al.² found that the iron compound shown in Figure 1 harvests sunlight through most of the visible region with subsequent excited state electron transfer to a TiO₂ semiconductor with efficiencies >90%. Such a breakthrough in efficiency is remarkable, particularly when one considers the decades of prior research that failed to accomplish anything even close. This breakthrough raises the question, are we on a path to solar cells that utilize iron? Before addressing this question, it is worthwhile to consider the impact an iron center has on a dye molecule.

Recently, a research group from the Center for Nanoscale Science and Technology at the National Institute of Standards and Technology (NIST), and the Massachusetts Institute of Technology in the United States has demonstrated a new type of quantum electro-optic phenomenon, whispering-gallery mode resonators.¹ The resonators are generated by a scanning tunneling microscope (STM) in proximity to graphene devices (Figure 1). On the basis of the quantum effect of electron tunneling, STM is a powerful technique to investigate the local electronic properties of both metallic and semiconducting systems with atomic resolution. Graphene, the most acclaimed material of the last decade, has enabled new horizons for STM research. The graphene surface can be directly probed by the scanning tip, whereas remaining chemically stable and clean even exposure to ambient air for days. Charged carriers in graphene can be readily tuned from holes to electrons using an external gate electrode. Furthermore, the charge carriers in graphene, often called Dirac particles, behave like electromagnetic waves, setting the stage for graphene to realize quantum electro-optic phenomena such as Veselago lensing² and Klein tunneling.³

Wu et al.¹ demonstrated a two-dimensional (2D) material-based laser that required only 1 W cm⁻² of pump power to reach the threshold limit. This value is low enough to be optically driven by a regular household light bulb! Reducing the power level for the onset of lasing action is a desirable goal in laser science. A series of design choices led to this breakthrough: (1) the 2D gain material exhibited high conversion efficiencies; and (2) the laser cavity—a photonic crystal cavity (PCC)—had a high quality factor (Figure 1).

Silicene is the silicon counterpart of graphene, that is, it consists of a single layer of Si atoms arranged in a hexagonal network. This new two-dimensional material, first predicted by theory, has been recently grown on different metallic surfaces.^{1, 2, 3} An obvious advantage of silicene (over graphene) for nanoelectronic applications is its better compatibility and expected integration with the existing Si nanotechnology platform. A new breakthrough on this material has been recently reported by Tao et al.,⁴ who have successfully fabricated the first silicene-based field effect transistors (FETs) operating at room temperature. Their success relies on the development of a layer transfer process, called ‘silicene-encapsulated delamination with native electrodes’ (SEDNE). This innovative process includes the following key steps: (1) epitaxial growth of silicene on Ag(111) thin films grown on mica substrates; (2) Al₂O₃ in situ encapsulation of the silicene layer, followed by its delamination transfer on a p⁺⁺Si/SiO₂ substrate; and (3) subsequent Ag source/drain contact formation by e-beam lithography. A resulting silicene-based FET, with the p⁺⁺Si substrate used as a back-gate contact, is shown in Figure 1a.

In the growing field of nanotechnology, there is an interest in developing hybrid organic–inorganic devices that have controllable electrical or magnetic properties.¹ Because of the nanoscale involved, the surface-to-volume ratio in these devices is large; hence, the devices can be controlled by varying their surface properties. Exerting control by using light is particularly attractive because making conventional hard electric contacts may be difficult due to size and material properties. The work of Suda et al.² presents a device in which superconductivity is controlled by light through the excitation of a gate made from spiropyran.³ Figure 1 schematically presents the device and its mode of operation (right panel) relative to that of a common field effect transistor (left panel). Spiropyran serves as the gate and is reversibly photoisomerized from a nonionic to a zwitterionic form. In its neutral form, no field is applied to the thin single crystal of κ-(BEDT-TTF)₂Cu[N(CN)₂]Br (κ-Br) (BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene). Upon photoexcitation with UV light, zwitterions are formed in the spiropyran film, and as a result, holes are injected into the κ-Br, converting it to a superconductor at low temperatures. Irradiation with visible light returns the film to its neutral state.

Recently, a new class of artificial materials known as metamaterials has emerged to manipulate light at the nanoscale. Contrary to natural materials, the physical properties of optical metamaterials are primarily dependent on the material structures rather than their chemical constituents. The structures forming the building blocks of the metamaterials are much smaller than the wavelength of light. By tailoring both effective electric permittivity and the magnetic permeability of metamaterials from a positive to a negative value, one can create negative refractive index metamaterials (NIMs).¹ Various 3D NIM structures have been previously reported,² but they are limited to small-scale samples of less than 1 mm. The unprecedented properties of metamaterials and their potential revolutionary applications, such as superlens imaging,³ remain a challenge to implement in practice due to the lack of innovative large-scale manufacturing methods. As a result, a scalable scheme that enables the large-area fabrication of 3D nanostructures must be developed.