Graphene’s success has shown that it is possible to create stable, single and few-atom-thick layers of van der Waals materials, and also that these materials can exhibit fascinating and technologically useful properties. Here we review the state-of-the-art of 2D materials beyond graphene. Initially, we will outline the different chemical classes of 2D materials and discuss the various strategies to prepare single-layer, few-layer, and multilayer assembly materials in solution, on substrates, and on the wafer scale. Additionally, we present an experimental guide for identifying and characterizing single-layer-thick materials, as well as outlining emerging techniques that yield both local and global information. We describe the differences that occur in the electronic structure between the bulk and the single layer and discuss various methods of tuning their electronic properties by manipulating the surface. Finally, we highlight the properties and advantages of single-, few-, and many-layer 2D materials in...

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Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene

The recent surge in graphene research has stimulated interest in the investigation of various 2-dimensional (2D) nanomaterials. Among these materials, the 2D boron nitride (BN) nanostructures are in a unique position. This is because they are the isoelectric analogs to graphene structures and share very similar structural characteristics and many physical properties except for the large band gap. The main forms of the 2D BN nanostructures include nanosheets (BNNSs), nanoribbons (BNNRs), and nanomeshes (BNNMs). BNNRs are essentially BNNSs with narrow widths in which the edge effects become significant; BNNMs are also variations of BNNSs, which are supported on certain metal substrates where strong interactions and the lattice mismatch between the substrate and the nanosheet result in periodic shallow regions on the nanosheet surface. Recently, the hybrids of 2D BN nanostructures with graphene, in the form of either in-plane hybrids or inter-plane heterolayers, have also drawn much attention. In particular, the BNNS–graphene heterolayer architectures are finding important electronic applications as BNNSs may serve as excellent dielectric substrates or separation layers for graphene electronic devices. In this article, we first discuss the structural basics, spectroscopic signatures, and physical properties of the 2D BN nanostructures. Then, various top-down and bottom-up preparation methodologies are reviewed in detail. Several sections are dedicated to the preparation of BNNRs, BNNMs, and BNNS–graphene hybrids, respectively. Following some more discussions on the applications of these unique materials, the article is concluded with a summary and perspectives of this exciting new field.

Advances in 2D boron nitride nanostructures: nanosheets, nanoribbons, nanomeshes, and hybrids with graphene

Spin mapping at the nanoscale and atomic scale

We demonstrate the ability to measure the energy required to flip the spin of single adsorbed atoms. A low-temperature, high-magnetic field scanning tunneling microscope was used to measure the spin excitation spectra of individual manganese atoms adsorbed on Al2O3 islands on a NiAl surface. We find pronounced variations of the spin-flip spectra for manganese atoms in different local environments.

https://science.sciencemag.org/content/sci/306/5695/466.full.pdf

Single-Atom Spin-Flip Spectroscopy

In molecular spintronics, the spin state of a molecule may be switched by changing the molecular structure Here, the spin of a single-molecule magnet is switched by applying an electric current using a scanning tunnelling microscope, which may aid in information coding at the single-molecule level

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Observation and electric current control of a local spin in a single-molecule magnet

The Kondo resonance of Co adatoms on the Cu(100) and Cu(111) surfaces has been studied by scanning tunneling spectroscopy. We demonstrate the scaling of the Kondo temperature ${T}_{\mathrm{K}}$ with the host electron density at the magnetic impurity. The quantitative analysis of the tunneling spectra reveals that the Kondo resonance is dominated by the Cu bulk electrons. While at the Cu(100) surface both tunneling into the hybridized localized state and into the substrate conduction band contribute to the Kondo resonance, the latter channel is found to be dominant for Cu(111).

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Kondo effect of single Co adatoms on Cu surfaces.

Electronic decoupling and templating of Co nanocluster arrays on the boron nitride nanomesh

The interaction of a magnetic impurity with the electrons of a non-magnetic host serves as a paradigm of many-body physics. The explanation of the resistivity minimum of dilute magnetic alloys at low temperatures and the development of appropriate theoretical tools to understand the rich phenomenology of the Kondo effect 1,2) are still shaping the research now done in heavy fermion systems and Hi-Tc superconductivity. 3) Even in the domain of the classical Kondo effect interest has been renewed when two different experimental approaches became possible: one was to study the Kondo effect in quantum dots which behave like artificial ‘‘spin impurities’’, 4–6) the other was the use of the scanning tunneling micrsoscope (STM) to study the electronic properties of single magnetic atoms on metal surfaces. 7,8) We present here results of the latter technique. The Kondo effect is due to the formation of a correlated singlet ground state at low temperatures in which the magnetic spin of the impurity is screened by a cloud of conduction band electrons interacting with the impurity. The formation of this state lowers the energy of the electronic system by the amount kBTK, where TK is the Kondo temperature of the system which ranges from sub-Kelvin to several hundred Kelvin. As function of temperature T transport coefficients like resistivity, specific heat, and susceptibility depend on T=TK, making TK the fundamental parameter of the low energy excitations of the many-electron problem. The special properties of the transport coefficients are caused by the formation of a resonance at the Fermi energy in the single particle density of states of the impurity. �

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M. Alexander Schneider

Papers

Kondo effect of single Co adatoms on Cu surfaces.

Electronic decoupling and templating of Co nanocluster arrays on the boron nitride nanomesh

Kondo Effect of Co Adatoms on Ag Monolayers on Noble Metal Surfaces