This CNMS Cryostat STM equipment is a home-made ultra-high vacuum (UHV) variable-temperature scanning tunneling microscope (STM), with molecular-beam epitaxy (MBE) growth capability. It has been optimized for deposition and growth using molecular precursors.
Science Overview
The CNMS Cryostat STM system consists of two UHV chambers, respectively as the MBE growth chamber and the STM characterization chamber. This system provides the ability to characterize in situ grown or cleaved materials in a carefully controlled environment without surface contamination.
The first chamber combines an argon ion (Ar+) sputtering gun, two K-cell evaporators and a manipulator stage, which can work at low temperature. With repeated cycles of Ar+ sputtering and annealing, single crystal surfaces, like Au and Cu, can be cleaned. The two-evaporator setup can enable the deposition of single molecules or two molecules at the same time. The low-temperature (LT) manipulator can also enable the LT deposition.
The second chamber of the system houses a home-made VT STM for atomically resolved characterization of the structural and electronic properties of surfaces. The STM head is cooled with continuous flow of liquid N2 or liquid He, with a temperature range of 20 K to 300 K.
Figure 1. Bottom-up synthesis. (a) Growth of armchair graphene nanoribbons with a seven-carbon width (7-aGNRs) from DBBA with a two-step annealing process respectively at 470 K and 670 K. (b): STM images of polymer islands. (c) High-resolution image of a single polymer. (d) STM images of 7-aGNRs. (e) High-resolution image of a single 7-aGNR.
Applications
In the pursuit of atomically precise and bottom-up fabrication of graphene-based electronics, graphene nanoribbons (GNRs) with a variety of widths, edge structures and heterojunctions have been synthesized with self-assembled molecular precursors on different catalytic metal substrates, such as Au, Ag and Cu. The cryostat STM system was used to grow and study the atomically precise bottom-up GNRs. The 10,10’-dibromo-9,9’-bianthryl (DBBA) molecules were adopted as the precursor to grow the 7-aGNRs on an Au (111) substrate in a two-step polymerization (at 470 K) and cyclodehydrogenation (at 670 K) process, as illustrated in Figure 1. When isolated with a layer of GNRs, the second layer polymer can only be partially converted to GNRs at the ends with thermal annealing, due to the absence of the catalytic effect of the Au substrate. However, with STM tip manipulations by injecting holes to the polymers, GNRs can be controllable fabricated at arbitrary sites, as shown in Figure 2. Nature Communications, 8, 14815 (2017).
Atomic precision GNR heterojunctions with different band alignments have a big potential to be used in GNR-based devices. With controllable tip-assisted polymer-to-GNR reactions, polymer/GNR/polymer heterojunctions can be achieved. Scanning tunneling spectroscopy (STS) measurements confirm a type-I band alignment, where the polymer has a bigger bandgap of 4.3 eV, and the 7-aGNR has a smaller bandgap of 2.5 eV. The large-bandgap polymer is not a good electrode for contacts to conventional metals when fabricating a device due to large Schottky barriers. Thus, it is important to synthesize HJs with narrow bandgaps or metallic and wide GNRs seamlessly connecting to a wide bandgap and narrow GNR. After further annealing of the 7-aGNRs at 770 K, 7-14 aGNR and 7-14-21 aGNR heterojunctions can be achieved. STS characterizations show that the 14-aGNR is quasi-metallic with a small bandgap of about 0.2 eV, while the 21-aGNR with a bandgap of about 0.7 eV. The staircase heterojunctions with atomically controlled seamless interfaces provide a promising interconnect to the semiconductor channel material, which would avoid Fermi-level pinning and a high Schottky barrier. Nano Lett. 17, 6241−6247 (2017).
Figure 2. STM tip assisted fabrication. Controllable conversion of polymer to GNR.
Specifications
General
- Samples must be UHV compatible, able to be mounted on a metal transfer plate and relatively flat
- Sample size up to 12×12 mm2
- In general, samples must be cleaned in situ to provide meaningful measurements
- Inserted sample surfaces can be prepared by cleaving, inert gas sputtering or heated in vacuum
Growth
- Sample stage temperature: 20 ~1000 K
- Two K-cell evaporators
STM/STS
- Home-made VT-STM/STS
- Temperatures from 20-300K
- STM requires a conductive path to ground
- Note that specific positioning of the STM tip on the sample (to less than ~250 µm) is not possible.
