How to keep graphene nanoribbons atomically perfect during transfer


Oct 05, 2025

Atomic imaging shows graphene nanoribbons retain atomic structure and adjustable electronic alignment after transfer to graphene, offering a clear route toward stable, reproducible nanoelectronic device integration.

(Nanowerk Spotlight) Every generation of electronics edges closer to the atomic scale, where the line between engineered materials and individual molecules disappears. At this frontier, the behavior of a device depends not only on its size but also on the precise arrangement of atoms. Researchers are learning how to shape and measure matter with single-atom precision, hoping to create faster and more efficient components that remain stable when reduced to the smallest possible dimensions. Graphene has become a central focus of this effort. It is a single layer of carbon atoms arranged in a hexagonal lattice, known for exceptional strength and electrical conductivity. Yet graphene on its own cannot act as a traditional semiconductor because it lacks a bandgap, the small energy separation that allows a transistor to switch current on and off. Cutting graphene into narrow strips only a few atoms wide changes that property. These strips, called graphene nanoribbons, develop a tunable bandgap that depends on their width and on the arrangement of atoms along their edges. In principle, that makes them suitable as molecular-scale electronic materials that can be customized with atomic accuracy. Their fragility, however, presents a major challenge. The electrical characteristics of a nanoribbon depend on perfect edge geometry, and even a single misplaced atom can alter how current flows through it. Chemists can now grow nearly flawless nanoribbons on smooth gold surfaces by linking carefully designed molecules under vacuum. On those surfaces, scanning tunneling microscopes can resolve each bond with atomic precision. The difficulty begins when the ribbons must be moved from gold to the practical materials used in real devices such as silicon or graphene. During transfer they can tear, curl, or react with traces of oxygen and solvents. Subtle defects invisible to most measurements can erase the properties that make the ribbons useful. Traditional inspection tools provide only a partial view. Raman spectroscopy and similar optical methods measure the overall quality of large collections of ribbons but cannot reveal what happens to individual ones after transfer. A sample may appear intact even if many ribbons have shortened or fused together. Without a direct way to see these changes, building consistent nanoribbon-based electronics remains uncertain and unpredictable. A study published in ACS Applied Nano Materials (“Atomic-Scale Imaging of Transferred Graphene Nanoribbons for Nanoelectronic Device Integration”) addresses this uncertainty. It uses scanning tunneling microscopy and spectroscopy to examine nanoribbons after they have been transferred onto graphene substrates, the same kind used in prototype electronic devices. By comparing their atomic structure and electronic states before and after transfer, the research clarifies what survives the process, what is lost, and how the underlying graphene influences the ribbons’ electronic behavior. This atomic-level perspective provides the clearest picture yet of how these precisely engineered materials behave once they are moved into the environments where they are meant to function. Transfer of 9-atom-wide armchair-edged graphene nanoribbons onto epitaxial graphene on SiC substrates Transfer of 9-atom-wide armchair-edged graphene nanoribbons (9-AGNRs) onto epitaxial graphene on SiC substrates. (a) Schematic illustration of the protocol used to transfer 9-AGNR grown on Au/Mica surfaces to EG. Once grown, the Au film is delaminated from the mica substrate and is picked up by the EG substrate. The Au film is then etched, leaving the GNRs on the EG substrate. Due to the solution processing steps performed under ambient conditions, the resulting surface is not UHV clean, and a high-temperature annealing (∼750 °C) in UHV is required to desorb the surface contaminants. (b) STM image of the GNRs as synthesized on Au/Mica (−1.5 V, 30 pA). (c) STM image of the GNRs transferred on EG (1.6 V, 10 pA). (d) Raman measurements of the GNRs as grown on Au/Mica (red) and after the transfer onto EG and subsequent UHV annealing (black). (click on image to enlarge) The investigation focuses on nine-atom-wide armchair graphene nanoribbons, a form already known for chemical stability and well-defined electronic properties. “Armchair” refers to the smooth edge pattern, while the width determines the size of the bandgap that controls current flow. The ribbons were first synthesized on gold using molecular precursors that couple together under heat to form extended carbon chains. The challenge lay in moving these delicate structures from gold to graphene without damaging their edges. Two transfer methods were tested. One used a polymer-free wet process in which a thin gold layer carrying the ribbons was floated on water and placed onto epitaxial graphene, which is graphene grown directly on a silicon carbide surface. The gold was then removed with an iodine-based etchant. The second method, called electrochemical bubble transfer, employed a temporary polymer support while hydrogen bubbles released the gold film from its base. Both aimed to keep the ribbons intact and clean. Because exposure to air leaves residues, the samples were heated in vacuum to about 750 degrees Celsius to restore a clean surface for microscopy. After cleaning, atomic-resolution images showed that most of the nine-atom-wide ribbons retained their original structure. The characteristic armchair edges were visible, along with small “bite” defects where a carbon ring was missing. A few fused ribbon ends appeared, probably formed during heating, but there was no sign of large-scale damage or oxidation. These observations confirmed that armchair ribbons remain chemically stable under the demanding conditions required to remove contamination. Raman spectroscopy supported this view. The key vibrational signal known as the radial breathing-like mode, which reflects the ribbon width, stayed at roughly 312 inverse centimeters before and after transfer. Other vibrational peaks associated with edge bonds became slightly broader, suggesting minor structural changes but not wholesale degradation. Comparisons with samples heated on gold showed that gold surfaces encourage far more fusion, likely because of catalytic effects that are absent on graphene. The contrast demonstrated that graphene provides a protective and chemically inert support for post-transfer cleaning. Although the atomic arrangement survived, the ribbons became shorter. Before transfer they averaged about twenty-six nanometers in length; afterward they measured around fifteen nanometers, with a wide spread from three to fifty. This shortening likely occurred through mechanical stress during the wet process or through fragmentation during annealing. Length matters because most prototype devices have contact separations near twenty nanometers, and only ribbons long enough to bridge that gap can conduct effectively. Optical techniques cannot detect such moderate length losses, which makes atomic-level imaging essential for realistic device assessment. The study then examined how the electronic structure of the ribbons changed when they rested on different forms of graphene. Scanning tunneling spectroscopy revealed two distinct peaks corresponding to the upper edge of the valence band and the lower edge of the conduction band. These features define the bandgap, which for ribbons on epitaxial graphene measured about 1.7 electronvolts, larger than the 1.4 electronvolts seen when ribbons remain on gold. The increase arises because graphene screens electronic charges less effectively than gold, allowing the intrinsic properties of the ribbon to dominate. When the same ribbons were transferred to quasi-freestanding epitaxial graphene, created by inserting hydrogen atoms between the graphene layer and its silicon carbide base, all energy levels shifted downward by roughly 0.2 electronvolts. This change reflected a higher work function, meaning electrons needed slightly more energy to escape the surface. The adjustment in level alignment affects how easily electrons or holes can move between ribbon and substrate. On epitaxial graphene the alignment was nearly symmetrical around the Fermi level, which marks the highest occupied energy state, implying that both types of charge carriers could be injected with similar efficiency. That symmetry could reduce contact resistance in future devices. The researchers also tested more complex and reactive nanoribbon structures. Ribbons incorporating cobalt–porphyrin units along zigzag edges partially decomposed during transfer, even though a few porphyrin cores survived. Raman spectroscopy could not detect clear signals from these samples, confirming that many structures had broken down. Another type of ribbon only seven atoms wide, designed to host topological quantum states, proved even less stable, showing no recognizable pattern in the microscope after transfer. These comparisons established that current transfer and cleaning procedures preserve robust armchair ribbons but destroy those with more reactive or open-shell designs. The experimental details described in the paper are as important as the images themselves. The authors outline how epitaxial and quasi-freestanding graphene are prepared on silicon carbide, the specific conditions for hydrogen intercalation, and the full sequence of both transfer routes. Each stage is documented with corresponding imaging and spectroscopy results, creating a template for laboratories that aim to reproduce the process. The combination of reproducible preparation and atomic-scale characterization sets a new reference point for nanoribbon research. Taken together, the findings close a critical gap between molecular synthesis and electronic integration. They confirm that nine-atom-wide armchair nanoribbons can be moved from gold to graphene without losing their defining atomic structure. They also show that graphene provides a nearly ideal platform for both imaging and electrical contact, revealing the true energy landscape that governs device performance. By quantifying the degree of shortening, identifying minor edge changes, and linking substrate work function to energy alignment, the study transforms an uncertain fabrication step into one that can be measured, compared, and optimized. This work strengthens the bridge between atomic-scale materials chemistry and functional nanoelectronics. It demonstrates that with careful handling, the precision achieved in surface synthesis can be preserved when materials are transferred into real devices. The approach offers a model for future studies of molecular semiconductors and other one-dimensional materials that depend on atomic perfection for their electronic behavior.


Michael Berger
By
– Michael is author of four books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology (2009),
Nanotechnology: The Future is Tiny (2016),
Nanoengineering: The Skills and Tools Making Technology Invisible (2019), and
Waste not! How Nanotechnologies Can Increase Efficiencies Throughout Society (2025)
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