Gold for depositing true 2D or atomically thin films
A novel method of producing ultrathin gold films offers great opportunities for optoelectronics.
Acombination of several key properties makes thin gold films a staple of photonics, plasmonics and metamaterials design. Gold conducts electricity well, is chemically stable, and has fairly low optical losses in the visible and near-infrared ranges. Ultrathin gold films at thinner than 10nm have prospects for designing flexible transparent electrodes, for use in solar cells, displays and touchscreens, photodetectors, LEDs, and waveguides. Fundamental science would also benefit from studying two-dimensional (2D) metals – which are still emerging – by being able to study the quantum effects in metal films.
The pursuit of ever-thinner metal films has the goal of depositing true 2D or atomically thin films. Today, materials with a layered crystal structure beyond graphite, such as molybdenum disulphide (MoS₂), lend themselves to routine exfoliation and synthesis. Research by the Moscow Institute of Physics and Technology, Russia, found that a monolayer of MoS₂ is a key to quasi-2D gold deposition. The study Ultrathin and ultrasmooth gold films on monolayer MoS₂ was supported by a Russian Science Foundation grant and the findings were reported in Advanced Materials Interfaces online in April 2019.
2D moly thins down gold films
Regardless of the technology used, the bane of ultrathin metal film deposition is what is known as the Volmer-Weber growth mode. For example, one common technology, known as electron beam evaporation, involves bombarding a sample of gold or other bulk metal with an electron beam under a high vacuum. The metal evaporates then precipitates into solid form, coating everything in the vacuum chamber with a thin metal film. However, it is energetically more favourable for the metal atoms to adhere to each other rather than to a silicon-based graphene, or glass substrate. The films exhibit a mode of growth that initially produces metal islands. By the time the gaps are filled and the film is continuous, it is about 20nm – too thick for optoelectronic applications.
To counteract the Volmer-Weber growth mode and obtain continuous films at lower thicknesses, adhesion layers may be introduced on the substrate. Materials used for this include titanium, chromium, nickel, platinum and germanium. Unfortunately, they adversely affect the optical and electrical properties of ultrathin films. Recently tested adhesion layers based on mercaptosilane and aminosilane are incompatible with pure silicon substrates and poorly compatible with the standard procedures for film lift-off. This motivated researchers to seek alternatives.
After initially experimenting with graphene, the group at the Moscow Institute of Physics and Technology found that MoS₂ produced excellent results as an adhesion layer, leading to thinner, smoother films. The standard gold film deposition procedure was modified by incorporating a monolayer of MoS₂ between the silicon-based substrate and the film. This compound seemed to be a reasonable candidate, because sulphur is among the very few elements that form stable bonds with gold.
The MoS₂ monolayer can be grown via chemical synthesis and then transferred to an arbitrary surface. First a silicon dioxide substrate is introduced into a furnace, then a reaction at elevated temperatures between molybdenum oxide and sulphur produces a MoS₂ coating on the substrate. This deposition technology is already established on an industrial scale. It is tunable so a monolayer will be deposited. After it has been grown, the 2D sheet of MoS₂ can be coated with a polymer matrix for easy handling. The substrate is then dissolved, and the matrix allows the monolayer to be moved elsewhere. Finally, the polymer needs to be dissolved.
Introducing this MoS₂ sheet as an adhesion layer results in higher quality ultrathin gold films obtained via standard deposition procedures using electron beam evaporation or other techniques. Because of the affinity gold atoms have for sulphur, the films grow in a more uniform manner. Since the gaps between islands disappear sooner, continuous films form at lower thicknesses. Those in our study are only 3-4nm thick. Although no further experiments have been conducted, the resulting hypothesis is that using an MoS₂ adhesion layer should work similarly with other deposition technologies and possibly with other metals, such as silver or copper.
Using MoS₂ results in thinner continuous films because of how this adhesion layer affects gold growth kinetics. Numerous energy-based parameters describe the growth kinetics for a metal. These are the adsorptions – the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface, then the energy of the metal adatom – an atom that lies on a crystal surface on the substrate, plus the metal cohesive energy, and the diffusion barrier. The ratio of the adsorption energy to the metal cohesive energy for an MoS₂ substrate and gold adatoms suggests that the growth kinetics of gold on MoS₂ should be similar to one on silicon dioxide (SiO₂) and governed by the Volmer–Weber mechanism. But the diffusion barrier for MoS₂ is large and limits the diffusion of metal adatoms on the surface, leading to a fine-grained gold film structure and a percolation threshold below 3nm.
The higher quality of ultrathin gold films on MoS₂ becomes evident when compared with their counterparts deposited on the conventionally used SiO₂ substrate. To be of use in optoelectronics applications, the gold films need to be ultrathin (less than 10nm) continuous, conductive and transparent. The research team demonstrated the superior properties of the films on MoS₂ by extensive structural, electrical and optical characterisation. The optical properties of the samples were studied via spectral ellipsometry, reporting for the first time the optical constants for ultrathin gold films.
Gold films grown on the surface of 2D MoS₂ have a continuous smooth morphology at thicknesses down to 3-4nm, while the films on SiO₂ start to percolate and form continuous gold structure only at about 6-8nm, depending on the deposition conditions. This results in quite different physical properties, including electrical and optical ones. The 3-9nm gold films on MoS₂ have electrical resistance in the range from 300-11 Ohms per square, which is comparable to the resistance of transparent electrodes used in modern electronic devices. On the contrary, gold films deposited on pure SiO₂ are nonconductive at thicknesses below 6-8nm because they are formed from isolated metal islands. Since the electrons can move freely under an electromagnetic field, the optical properties of our gold films on MoS₂ are similarly advantageous. At 3-4nm, they exhibit low optical losses as well as Drude plasmonic response, which is required for optoelectronics.
Optoelectronics and metamaterials
Thin gold films, down to about 20nm, are routinely grown on silicon-based substrates and used in microelectronics and biosensors. However, it would take thinner films that are smaller than 10nm in size to enable the ultrathin, flexible and transparent electrodes, with applications including folding displays, e-paper, e-clothing, and lenses with built-in electronics. By depositing 3-4nm continuous films, the research team has showed that using MoS₂ as an adhesion layer in film manufacturing can make these exciting applications a reality. A particularly intriguing possibility would be to use the electrodes in neural interfaces for treating patients or to integrate the nervous system of a living being with electronic devices.
Thin metal films can also replace the conductive metal oxides, such as indium tin oxide (ITO). These are widely used in the microelectronics industry but are poorly compatible with flexible electronics. ITO in particular is quite expensive because of the rare element indium.
By dropping metal oxides in favour of ultrathin metal films in display equipment, flexibility can be achieved without sacrificing conductivity or transparency.
High-quality ultrathin metal films are useful for metamaterials design. By integrating quasi-2D gold into the multilayer stacks of 2D materials known as van der Waals (vdW) heterostructures, the latter gain a new facet. Previously, vdW heterostructures could incorporate semiconductors, dielectrics and semimetals. Now, bulk metals thinned down to several nanometres are a valid option. With more 2D materials in the toolkit, metamaterial properties can be tailored more efficiently to their specific applications in optoelectronics, photonics, plasmonics and elsewhere. For example, additional nanostructuring of thin metal layers enables light manipulation at the subwavelength scale. Also, one can tune the optical properties of multilayer optical systems, including those with negative refractive index. Such optical metamaterials open up a new avenue for the development of novel optical devices with applications ranging from medical technology to aerospace engineering.
The researchers expect this to be the beginning of quasi-2D metal science. With this new technology, it is possible to discuss the prospects they hold for flexible and transparent electronics and the data can help researchers model devices and metamaterials. Since an MoS₂ monolayer can be transferred to virtually any surface, the possibilities are endless for using this technology in optoelectronics and new materials design. It is hoped these quasi-2D metal materials will soon be in production.
As far as the research project, the team has obtained promising results working on films of several metals other than gold at thicknesses of 3-4nm. The next step is to further thin them to 1-2nm, with the ultimate goal of producing films of non-layered materials that are 2-3 atomic layers thick.
*Yury Stebunov is Senior Researcher and Aleksey Arsenin is Director of Center for Photonics and 2D Materials, at Moscow Institute of Physics and Technology, Russia.