Graphene at sub-nanometer scale unlocks intense quantum light interactions

(Nanowerk Spotlight) Since its isolation in 2004, graphene has captivated researchers with its roster of incredible properties from mechanical strength surpassing steel to electrical conductivity rivaling silver. Yet the often hyped “wonder” material has so far failed to meet its immense promise revolutionizing industries from electronics to energy storage.
Scalable manufacturing and fine-tuning graphene's intrinsic qualities through structure and interfaces remain monumental challenges. If these hurdles could be cleared, devices leveraging graphene's exotic physics could power new paradigms in computing, photonics, chemistry and more.
Recent advancements, reported in Advanced Materials ("Tailoring Graphite into Subnanometer Graphene"), detail an innovative top-down approach to fracture graphene sheets smaller than a single nanometer wide. With such precise atomic-scale tailoring, graphene exhibits intense light-matter interactions ranging from room-temperature superconductivity signatures to hot carrier harvesting not before possible.
All-physical top–down method for production of sub-1 nm graphene
All-physical top–down method for production of sub-1 nm graphene. a) Schematic illustration of the fabrication process. b) SEM and TEM images showing the well-controlled stepwise downsizing from graphite to graphene subnanometer materials. (Reprinted with permission by Wiley-VCH Verlag)
Dubbed graphene subnanometer materials (GSNs), these atomically thin graphene splinters exhibit exotic optoelectronic behaviors from ultrabright photoluminescence to extreme nonlinear light absorption. The fracturing process also grants reproducible control over final GSN structures. With further development, devices and systems leveraging intense light-matter interactions in GSNs could power next-gen photonic circuits, optical switches, light detection, and more.
Graphene slices measuring hundreds of layers thick still outstrip applications demanding single or few layer samples. Top-down exfoliation that chips away layers from graphite particles has improved but achieving flakes below 10 nanometers wide remains scarce. Such ultra-narrow graphene unlocks quantum and edge effects that enormously magnify properties. However direct physical grinding methods tended to produce highly heterogeneous debris.
Chemical synthesis approaches reliably generate atom precise nanoclusters and complexes. However, bottom-up construction cannot replicate graphene’s pristine honeycomb lattice and exotic electromagnetic behaviors stemming from band structure. Purely physical fracturing preserves these innate characteristics while accessing atomic dimensions difficult for current top-down techniques.
Recent research demonstrates producing graphene sheets around 3 nanometers in size via an alternate ball milling approach. However, further reducing dimensions below 1 nm remains extremely challenging. Extending mechanical exfoliation techniques to slice down to single lattice dimensions pushes against fundamental limits of mechanical pulverization.
The new work achieves this long-sought goal of sub-nanometer slicing via an optimized ball milling technique. iteratively employing a tag team of microbead assemblies with differing particle sizes, hardness, and collision dynamics fractured graphite bits down nearly to individual carbon atoms. Shock propagation through the milling matrix continually refreshed with minimal thermal effects or contamination to maximize slicing efficiency.
Starting with crystalline graphite flakes, initial coarse grinding with millimeter scale beads sheared bulk samples into platelet stacks tens of nanometers thick. An intermediate milling phase substituted hard silica microspheres just 450 nanometers wide alongside the large grinding beads. The pairing battered particles laterally, ripping arrays into progressively thinner and smaller sheets.
For the finale, much smaller 100 nm beads replaced some microspheres, approximating graphene’s innate atomic dimensions. Surface interactions, imparted kinetic energy, and contact pressures climbed exponentially compared to previous steps. Such extreme mechanical duress finally chopped stubborn layers to pieces 0.54 nm in diameter on average, evidenced by atomic force microscopy. Raman spectroscopy confirmed the hexagonal network fractured completely through the single-atom terminus width of graphene’s crystalline lattice.
This precisely controllable process generated GSN samples at remarkable scale, yielding gram batches from bulk graphite. Solutions dispersing shredded graphene remained stable for over a week with no discernible precipitation or aggregation. Such scalability and processability bode well for eventual commercial applications.
GSNs displayed intensely amplified light-matter interactions reflective of transient quantum states and particle-in-a-box confinement stemming from the atomically narrow topology. Without conductive pathways along in-plane and stacking dimensions, excited charge carriers piled up and lingered much longer around GSN edges as visualized through transient absorption spectroscopy.
Photoluminescence excitations readily triggered broadband visible wavelength emission, glowing extraordinarily brighter than quantum-scale samples. Researchers suggest exposed carbon bonds may introduce surface defect sites that radiatively relax. They also noted green spectra unique to GSN dispersions in NMP solvent, implying certain interactions selectively modulate relaxation channels. Alongside intensity, merely tuning solvent polarity dynamically switched output colors from green to blue.
But exponential enhancements to nonlinear absorption especially stand to transform optical systems by enabling all-optical control of light with light. At merely half the beam energy required by previous 2D materials, GSN films measurably attenuated intense laser irradiation, transitioning from virtually transparent to opaque. Researchers attribute such performance leaps to the preponderance of photon-capturing edges. Light-carving layers for terahertz beams could soon result.
This breakthrough technique to reliably splinter pristine graphene to individual atoms unlocks hitherto inaccessible nanostructure regimes. With optimization, diamond and analogous elemental lattices may also yield to extreme fracturing into exotic allotropes. Beyond shining new light on fundamental limits governing mechanical destruction of chemical bonds, however, GSNs uniquely bridge structure and function. By tampering with graphene at the very atomic level, intricate means to handle and channel light open to ultrathin optoelectronics, telecommunications, and spectroscopy.
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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