Antonio Polimeni, "The Hydrogen Route to the Engineering of the Physical Properties of Semiconductors at the Nanoscale", Dip. di Fisica, [chiamata]
In 1959, Richard Feynman gave a visionary lecture entitled “There’s Plenty of Room at the Bottom” [1], wherein he raised a fundamental question: ‘What could we do with layered structures with just the right layers? What would the properties of materials be if we could really arrange the atoms the way we want them?’.
Since Feynman’s inspiring vision, scientists put a great effort into the search for tailormade materials with controllable electronic, optical, magnetic, mechanical … properties. Soon, it became clear that semiconductors are an optimal material platform for achieving such a vision. Nowadays, thanks to the enormous progress in the synthesis and lithographic processing of semiconductors, it is possible to assemble different materials into nanometre-sized superstructures (or nanostructures) with engineerable physical and chemical properties. Those properties are continually at the origin of many unexpected and fascinating physical phenomena, as well as of ever smaller, faster and cost-effective devices.
Here, we review the creation and engineering of nanostructures thanks to the breaking and stretching of crystal bonds caused by hydrogen incorporation in two different classes of semiconductors.
In the first instance, we consider the effect of controlled hydrogen diffusion in nitrogen-dilute Ga(AsN). The latter is a highly mismatched alloy, wherein N atom incorporation narrows dramatically the host GaAs band gap. In Ga(AsN), hydrogen breaks the covalent bonds of N atoms in the crystal and passivates their chemical activity via the formation of N-2H complexes, acting as a modern Philosopher’s Stone. By exploiting electron-beam lithography, we control N-2H complex formation with few tens nanometre resolution and thus create size- and site-controlled quantum structures able to emit non-classical light.
In the second instance, we describe the effects of hydrogen diffusion in van der Waals (vdW) crystals that are layered lattices, in which two-dimensional crystal planes are stacked together by vdW forces. The most illustrative case of this class of materials is graphite, being formed by graphene planes (having notoriously zero band gap and featuring a metallic behaviour). Here, we consider vdW crystals with semiconducting properties, specifically transition metal dichalcogenides (or TMDs), which are presently among the most investigated vdW crystals thanks to their excellent optical properties. We show how hydrogen breaks vdW bonds by the formation of crystal bubbles filled with highly pressurised molecular hydrogen and featuring one atomic plane thickness. The bubble formation process, fascinating per se, can be lithographically controlled over an unprecedented scale ranging from few tens of nanometre to tens of microns. The very large strain fields engendered in the bubbles induce dramatic changes in the material’s electronic properties and prompt a plethora of intriguing effects.
[1] R. P. Feynman, There’s Plenty of Room at the Bottom, Eng. Sci. 23, 22 (1960).
