Wave and Particle in Molecular Interference Lithography

The wave-particle duality of massive objects is a cornerstone of quantum physics and a key property of many modern tools such as electron microscopy, neutron diffraction or atom interferometry. Here we report on the first experimental demonstration of quantum interference lithography with complex molecules. Molecular matter-wave interference patterns are deposited onto a reconstructed Si(111) $7\times 7$ surface and imaged using scanning tunneling microscopy. Thereby both the particle and the quantum wave character of the molecules can be visualized in one and the same image. This new approach to nanolithography therefore also represents a sensitive new detection scheme for quantum interference experiments.

Figure 1

Molecular interference lithography combines coherent molecule wave propagation with surface deposition and scanning probe microscopy. The furnace emits a thermal fullerene beam, velocity selected to $\Delta v/v=5\%$, which passes two SiN gratings ($d=257.40\mathrm{nm}$) separated by $L=12.5\mathrm{mm}$. A prepared silicon sample is placed in distance $L$ behind $G_2$. The molecular deposit is imaged with single molecule resolution using scanning tunneling microscopy in a separate UHV chamber.

Figure 2

STM image of a reconstructed Si (111) $7\times 7$ surface, covered with immobilized individual fullerenes. Area of this image: $33\times 36{\mathrm{nm}}^2$; tunneling current $I=0.2\mathrm{nA}$; sample bias voltage $U=+2\mathrm{V}$; temperature $T=30\mathrm{K}$. A close up on two fullerenes (see inset) gives a glimpse on the internal ring structure of ${\mathrm{C}}_{60}$ (see also [28]).

Figure 3

(a) The processed STM image (see text) reveals both the particle nature and the quantum wave nature of the surface-deposited fullerene molecules in one and the same image. (b) A vertical sum over (a) yields the interference curve. The fringe visibility amounts to 36%. The error bars represent the $\sqrt{N}$ noise related to the small number $N$ of particles per bin.