Writing Conductive Lines with Hot Tips The interface within devices between conductors, semiconductors, and insulators is usually created by stacking patterned layers of different materials. For flexible electronics, it can be advantageous to avoid this architectural constraint. Graphene oxide, formed by chemical exfoliation of graphite, can be reduced to a more conductive form using chemical reductants. Wei et al. (p. 1373 ) now show that layers of graphene oxide can also be reduced using a hot atomic force microscope tip to create materials comparable to those of organic conductors. This process can create patterned regions (down to 12 nanometers in width) that differ in conductivity by up to four orders of magnitude.

We present the first microscopic transport study of epitaxial graphene on SiC using an ultrahigh vacuum four-probe scanning tunneling microscope. Anisotropic conductivity is observed that is caused by the interaction between the graphene and the underlying substrate. These results can be explained by a model where charge buildup at the step edges leads to local scattering of charge carriers. This highlights the importance of considering substrate effects in proposed devices that utilize nanoscale patterning of graphene on electrically isolated substrates.

Holes confined in single InAs quantum dots have recently emerged as a promising system for the storage or manipulation of quantum information. These holes are often assumed to have only heavy-hole character and further assumed to have no mixing between orthogonal heavy hole spin projections (in the absence of a transverse magnetic field). The same assumption has been applied to InAs quantum dot molecules formed by two stacked InAs quantum dots that are coupled by coherent tunneling of the hole between the two dots. We present experimental evidence of the existence of a hole spin mixing term obtained with magneto-photoluminescence spectroscopy on such InAs quantum dot molecules. We use a Luttinger spinor model to explain the physical origin of this hole spin mixing term: misalignment of the dots along the stacking direction breaks the angular symmetry and allows mixing through the light-hole component of the spinor. We discuss how this novel spin mixing mechanism may offer new spin manipulation opportunities that are unique to holes.

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In a network of quantum dots 1 embedded in a semiconductor structure, no two are the same, and so their individual and collective properties must be measured after fabrication. Here, we demonstrate a 'level anti-crossing spectroscopy' (LACS) technique in which the ladder of orbital energy levels of one quantum dot is used to probe that of a nearby quantum dot. This optics-based technique can be applied in situ to a cluster of tunnel-coupled dots, in configurations similar to that predicted for new photonic or quantum information technologies 2-5 . Although the lowest energy levels of a quantum dot are arranged approximately in a shell structure 6-10 , asymmetries or intrinsic physics-such as spin-orbit coupling for holesmay alter level splittings significantly 11 . We use LACS on a diatomic molecule composed of vertically stacked InAs/GaAs quantum dots and obtain the excited-state level diagram of a hole with and without extra carriers. The observation of excited molecular orbitals, including σ and π bonding states, provides fresh opportunities in solid-state molecular physics. Combined with atomic-resolution microscopy and electronicstructure theory for typical dots, the LACS technique could also enable 'reverse engineering' of the level structure and the corresponding optical response 12 .To begin, we recall the previously established spectroscopic features of quantum dot molecules (QDMs). We use the recombination of an electron-hole pair (that is, an exciton X 0 ) as an indicator for the state of the hole. In these structures, the electron is localized in the bottom dot (B dot) over the entire electric field range [13][14][15][16] . Recombination with the hole within the same dot leads to an intense spectral line 1 0 1 0 in the photoluminescence spectrum ( Fig. 1). As the electric field is scanned, this intradot transition goes through the first of a series of anticrossings at a field that we take as F = 0. Here, there is a resonance with the lowest interdot transition, 1 0 0 1 , in which the hole is in the top-dot (T-dot) ground state. The interdot transition energy exhibits a strong field dependence (Stark shift), caused by a change in the relative level alignment between the two dots with electric field. The strength of the shift is determined by the dot separation. Its slope ( E/ F = 0.955 meV kV −1 cm) provides a built-in calibration for the conversion between electric field ( F) and energy ( E). This first resonance arises from the coherent tunnelling of the hole between the 's shells' of the two dots (B0 and T0), and the corresponding formation of a bonding and an antibonding molecular state. Such resonances between the ground states of two quantum dots have been studied intensely in recent publications [13][14][15][16][17][18][19][20][21][22][23] . Energy (meV) B-T-dot T0 CB VB 1 2 3 4 Intradot B0 Intradot Interdot 1 0 0 1 ΔE T0 T1 T2 T3 T4 1 1,252 1,253 ΔF --( ) 1 0 1 0 --( ) 1 0 1 1 --( ) 0 2 0 4 0 -3.0 -2.5 -2.0 -3.5Relative electric field (kV cm -1 )Applied bias (V) Figure 1 Principle of LACS. The ...