Single atoms and metal nanoclusters anchored to graphene vacancies.

Fabricating dispersed single atoms and size-controlled metal nanoclusters remains a difficult challenge due to sintering. Here, we demonstrate that atoms and clusters can be immobilized using atomically clean defect-engineered graphene as the matrix. The graphene is first cleaned of surface contamination with laser heating, after which low-energy Ar irradiation is used to create spatially well-separated vacancies into it. Metal atoms are then evaporated either via thermal or ebeam evaporation onto graphene, where they diffuse until being trapped into a vacancy. The density of embedded structures can be controlled through irradiation dose, and the size of the structures through evaporation time. The resulting structures are confirmed through atomic-resolution scanning transmission electron microscopy and electron energy loss spectroscopy. We demonstrate here incorporation of Al, Ti, Fe, Ag and Au single atoms or nanoclusters, but the method should work equally well for other elements.

Two-dimensional few-atom noble gas clusters in a graphene sandwich.

The van der Waals atomic solids of noble gases on metals at cryogenic temperatures were the first experimental examples of two-dimensional systems. Recently, such structures have also been created on surfaces under encapsulation by graphene, allowing studies at elevated temperatures through scanning tunnelling microscopy. However, for this technique, the encapsulation layer often obscures the arrangement of the noble gas atoms. Here we create Kr and Xe clusters in between two suspended graphene layers, and uncover their atomic structure through transmission electron microscopy. We show that small crystals (N < 9) arrange on the basis of the simple non-directional van der Waals interaction. Larger crystals show some deviations, possibly enabled by deformations in the encapsulating graphene lattice. We further discuss the dynamics of the clusters within the graphene sandwich, and show that although all the Xe clusters with up to N ≈ 100 remain solid, Kr clusters with already N ≈ 16 turn occasionally fluid under our experimental conditions (under a pressure of ~0.3 GPa). This study opens a way for the so-far unexplored frontier of encapsulated two-dimensional van der Waals solids with exciting possibilities for fundamental condensed-matter physics research and possible applications in quantum information technology.

Beam-driven Dynamics of Aluminium Dopants in Graphene.

Substituting heteroatoms into graphene can tune its properties for applications ranging from catalysis to spintronics. The further recent discovery that covalent impurities in graphene can be manipulated at atomic precision using a focused electron beam may open avenues towards sub-nanometer device architectures. However, the preparation of clean samples with a high density of dopants is still very challenging. Here, we report vacancy-mediated substitution of aluminium into laser-cleaned graphene, and without removal from our ultra-high vacuum apparatus, study their dynamics under 60 keV electron irradiation using aberration-corrected scanning transmission electron microscopy and spectroscopy. Three- and four-coordinated Al sites are identified, showing excellent agreement with ab initio predictions including binding energies and electron energy-loss spectrum simulations. We show that the direct exchange of carbon and aluminium atoms predicted earlier occurs under electron irradiation, although unexpectedly it is less probable than the same process for silicon. We also observe a previously unknown nitrogen-aluminium exchange that occurs at Al─N double-dopant sites at graphene divacancies created by our plasma treatment.

Toward Exotic Layered Materials: 2D Cuprous Iodide.

Heterostructures composed of 2D materials are already opening many new possibilities in such fields of technology as electronics and magnonics, but far more could be achieved if the number and diversity of 2D materials were increased. So far, only a few dozen 2D crystals have been extracted from materials that exhibit a layered phase in ambient conditions, omitting entirely the large number of layered materials that may exist at other temperatures and pressures. This work demonstrates how such structures can be stabilized in 2D van der Waals (vdw) stacks under room temperature via growing them directly in graphene encapsulation by using graphene oxide as the template material. Specifically, an ambient stable 2D structure of copper and iodine, a material that normally only occurs in layered form at elevated temperatures between 645 and 675 K, is produced. The results establish a simple route to the production of more exotic phases of materials that would otherwise be difficult or impossible to stabilize for experiments in ambient.

Single Indium Atoms and Few-Atom Indium Clusters Anchored onto Graphene via Silicon Heteroatoms.

Single atoms and few-atom nanoclusters are of high interest in catalysis and plasmonics, but pathways for their fabrication and placement remain scarce. We report here the self-assembly of room-temperature-stable single indium (In) atoms and few-atom In clusters (2-6 atoms) that are anchored to substitutional silicon (Si) impurity atoms in suspended monolayer graphene membranes. Using atomically resolved scanning transmission electron microscopy (STEM), we find that the symmetry of the In structures is critically determined by the three- or fourfold coordination of the Si "anchors". All structures are produced without electron-beam induced materials modification. In turn, when activated by electron beam irradiation in the STEM, we observe in situ the formation, restructuring, and translation of the Si-anchored In structures. Our results on In-Si-graphene provide a materials system for controlled self-assembly and heteroatomic anchoring of single atoms and few-atom nanoclusters on graphene.

Atomic-Level Structural Engineering of Graphene on a Mesoscopic Scale.

Structural engineering is the first step toward changing properties of materials. While this can be at relative ease done for bulk materials, for example, using ion irradiation, similar engineering of 2D materials and other low-dimensional structures remains a challenge. The difficulties range from the preparation of clean and uniform samples to the sensitivity of these structures to the overwhelming task of sample-wide characterization of the subjected modifications at the atomic scale. Here, we overcome these issues using a near ultrahigh vacuum system comprised of an aberration-corrected scanning transmission electron microscope and setups for sample cleaning and manipulation, which are combined with automated atomic-resolution imaging of large sample areas and a convolutional neural network approach for image analysis. This allows us to create and fully characterize atomically clean free-standing graphene with a controlled defect distribution, thus providing the important first step toward atomically tailored two-dimensional materials.

Tailoring Electronic and Magnetic Properties of Graphene by Phosphorus Doping.

The electronic and magnetic properties of graphene can be modulated by doping it with other elements, especially those with a different number of valence electrons. In this article, we first provide a three-dimensional reconstruction of the atomic structure of a phosphorus substitution in graphene using aberration-corrected scanning transmission electron microscopy. Turning then to theoretical calculations based on the density functional theory (DFT), we show that doping phosphorus in various bonding configurations can induce magnetism in graphene. Our simulations reveal that the electronic and magnetic properties of P-doped (Gr-P) and/or phosphono-functionalized graphene (Gr-PO3H2) can be controlled by both the phosphorus concentration and configurations, ultimately leading to ferromagnetic (FM) and/or antiferromagnetic (AFM) features with the transition temperature up to room temperature. We also calculate core-level binding energies of variously bonded P to facilitate X-ray photoelectron spectroscopy-based identification of its chemical form present in P-doped graphene-based structures. These results may enable the design of graphene-based organic magnets with tailored properties for future magnetic or spintronic applications.

Scalable growth of single-walled carbon nanotubes with a highly uniform structure.

Here, a scalable floating catalyst chemical vapor deposition (FCCVD) method is developed for the production of single-walled carbon nanotubes (SWCNTs) with a controlled structure. For the first time, water is used as the growth promoter in the FCCVD process to modulate the growth of SWCNTs. At an optimum water concentration of ca. 115 ppm, the water-assisted FCCVD process synthesizes SWCNTs with a significantly narrow chirality distribution. In particular, the proportion of (9,8) and (8,7) semiconducting tubes was dramatically enhanced to 45% with 27% of the (9,8) tube in the end product. This is attributed to the changes in both the SWCNT diameter and the chiral angle. The experiment results and accurate quantum chemical molecular dynamics simulations show that the addition of water affects the nucleation and the size distribution of nanoparticle catalysts, thus resulting in the growth of SWCNTs with a highly uniform structure. This direct and continuous water-assisted FCCVD provides the possibility for the mass production of high-quality SWCNTs with a controlled structure.

Transformation and Evaporation of Surface Adsorbents on a Graphene "Hot Plate".

Dynamic surface modification of suspended graphene at high temperatures was directly observed with in situ scanning transmission electron microscopy (STEM) measurements. The suspended graphene devices were prepared on a SiN membrane substrate with a hole so that STEM observations could be conducted during Joule heating. Current-voltage characteristics of suspended graphene devices inside the STEM chamber were measured while monitoring and controlling the temperature of graphene by estimating the electrical power of the devices. During the in situ STEM observation at high temperatures, residual hydrocarbon adsorbents that had remained on graphene effectively evaporated creating large, atomically clean graphene areas. At other places, dynamic changes in the shape, position, and orientation of adsorbents could be directly observed. The temperature of the suspended graphene sample was estimated to reach up to 2000 K during the experiment, making graphene an efficient high-temperature micrometer-sized electron-transparent hot plate for future experiments in microscopes.

Enhanced Tunneling in a Hybrid of Single-Walled Carbon Nanotubes and Graphene.

Transparent and conductive films (TCFs) are of great technological importance. Their high transmittance, electrical conductivity, and mechanical strength make single-walled carbon nanotubes (SWCNTs) a good candidate for the raw material for TCFs. Despite the ballistic transport in individual SWCNTs, electrical conductivity of SWCNT networks is limited by low efficiency of charge tunneling between the tube elements. Here, we demonstrate that the nanotube network sheet resistance at high optical transmittance is decreased by more than 50% when fabricated on graphene. This is a comparable improvement as that obtained through gold chloride (AuCl3) doping. However, while Raman spectroscopy reveals substantial changes in spectral features of AuCl3 doped nanotubes, this does not occur with graphene. Instead, temperature-dependent transport measurements indicate that a graphene substrate reduces the tunneling barrier heights, while its parallel conductivity contribution is almost negligible. Finally, we show that combining the graphene substrate and AuCl3 doping, brings the SWCNT thin film sheet resistance down to 36 Ω/□.

Silicon Substitution in Nanotubes and Graphene via Intermittent Vacancies.

The chemical and electrical properties of single-walled carbon nanotubes (SWCNTs) and graphene can be modified by the presence of covalently bound impurities. Although this can be achieved by introducing chemical additives during synthesis, it often hinders growth and leads to limited crystallite size and quality. Here, through the simultaneous formation of vacancies with low-energy argon plasma and the thermal activation of adatom diffusion by laser irradiation, silicon impurities are incorporated into the lattice of both materials. After an exposure of ∼1 ion/nm2, we find Si-substitution densities of 0.15 nm-2 in graphene and 0.05 nm-2 in nanotubes, as revealed by atomically resolved scanning transmission electron microscopy. In good agreement with predictions of Ar irradiation effects in SWCNTs, we find Si incorporated in both mono- and divacancies, with ∼2/3 being of the first type. Controlled inclusion of impurities in the quasi-1D and -2D carbon lattices may prove useful for applications such as gas sensing, and a similar approach might also be used to substitute other elements with migration barriers lower than that of carbon.

Atomic-Scale Deformations at the Interface of a Mixed-Dimensional van der Waals Heterostructure.

Molecular self-assembly due to chemical interactions is the basis of bottom-up nanofabrication, whereas weaker intermolecular forces dominate on the scale of macromolecules. Recent advances in synthesis and characterization have brought increasing attention to two- and mixed-dimensional heterostructures, and it has been recognized that van der Waals (vdW) forces within the structure may have a significant impact on their morphology. Here, we suspend single-walled carbon nanotubes (SWCNTs) on graphene to create a model system for the study of a 1D-2D molecular interface through atomic-resolution scanning transmission electron microscopy observations. When brought into contact, the radial deformation of SWCNTs and the emergence of long-range linear grooves in graphene revealed by the three-dimensional reconstruction of the heterostructure are observed. These topographic features are strain-correlated but show no sensitivity to carbon nanotube helicity, electronic structure, or stacking order. Finally, despite the random deposition of the nanotubes, we show that the competition between strain and vdW forces results in aligned carbon-carbon interfaces spanning hundreds of nanometers.

Buckyball sandwiches.

Two-dimensional (2D) materials have considerably expanded the field of materials science in the past decade. Even more recently, various 2D materials have been assembled into vertical van der Waals heterostacks, and it has been proposed to combine them with other low-dimensional structures to create new materials with hybridized properties. We demonstrate the first direct images of a suspended 0D/2D heterostructure that incorporates C60 molecules between two graphene layers in a buckyball sandwich structure. We find clean and ordered C60 islands with thicknesses down to one molecule, shielded by the graphene layers from the microscope vacuum and partially protected from radiation damage during scanning transmission electron microscopy imaging. The sandwich structure serves as a 2D nanoscale reaction chamber, allowing the analysis of the structure of the molecules and their dynamics at atomic resolution.

Dry and Direct Deposition of Aerosol-Synthesized Single-Walled Carbon Nanotubes by Thermophoresis.

Single-walled carbon nanotubes (SWCNTs) show great potential as an active material in electronic and photonic devices, but their applicability is currently limited by shortcomings in existing deposition methods. SWCNTs can be dispersed from liquid solutions; however, their poor solubility requires the use of surfactants and ultrasonication, causing defects and degradation in device performance. Likewise, the high temperatures required by their chemical vapor deposition growth limit substrates on which SWCNTs can be directly grown. Here, we present a systematic study of the direct deposition of pristine, aerosol-synthesized SWCNTs by thermophoresis. The density of the deposited nanotube film can be continuously adjusted from individual, separated nanotubes to multilayer thin films by changing the deposition time. Depending on the lateral flow inside the thermophoretic precipitator, the angular distribution of the deposited SWCNT film can be changed from uniform to nonuniform. Because the substrate is kept at nearly ambient temperature, deposition can be thus carried out on practically any flat substrate with high efficiencies close to unity. The thermophoretic terminal velocity of SWCNTs, determined by aerosol loss measurements, is found to be approximately one-third of the usual prediction in the free molecular regime and shows a weak dependence on the nanotube diameter. As a demonstration of the applicability of our technique, we have used thermophoretic deposition in the fabrication of carbon nanotube thin-film transistors with uniform electrical properties and a high, over 99.5%, yield.

Photon-Pair Generation with a 100 nm Thick Carbon Nanotube Film.

Nonlinear optics based on bulk materials is the current technique of choice for quantum-state generation and information processing. Scaling of nonlinear optical quantum devices is of significant interest to enable quantum devices with high performance. However, it is challenging to scale the nonlinear optical devices down to the nanoscale dimension due to relatively small nonlinear optical response of traditional bulk materials. Here, correlated photon pairs are generated in the nanometer scale using a nonlinear optical device for the first time. The approach uses spontaneous four-wave mixing in a carbon nanotube film with extremely large Kerr-nonlinearity (≈100 000 times larger than that of the widely used silica), which is achieved through careful control of the tube diameter during the carbon nanotube growth. Photon pairs with a coincidence to accidental ratio of 18 at the telecom wavelength of 1.5 µm are generated at room temperature in a ≈100 nm thick carbon nanotube film device, i.e., 1000 times thinner than the smallest existing devices. These results are promising for future integrated nonlinear quantum devices (e.g., quantum emission and processing devices).

Toward the Limits of Uniformity of Mixed Metallicity SWCNT TFT Arrays with Spark-Synthesized and Surface-Density-Controlled Nanotube Networks.

We report the fabrication of thin film transistors (TFTs) from networks of nonbundled single-walled carbon nanotubes with controlled surface densities. Individual nanotubes were synthesized by using a spark generator-based floating catalyst CVD process. High uniformity and the control of SWCNT surface density were realized by mixing of the SWCNT aerosol in a turbulent flow mixer and monitoring the online number concentration with a condensation particle counter at the reactor outlet in real time. The networks consist of predominantly nonbundled SWCNTs with diameters of 1.0-1.3 nm, mean length of 3.97 µm, and metallic to semiconducting tube ratio of 1:2. The ON/OFF ratio and charge carrier mobility of SWCNT TFTs were simultaneously optimized through fabrication of devices with SWCNT surface densities ranging from 0.36 to 1.8 µm(-2) and channel lengths and widths from 5 to 100 µm and from 100 to 500 µm, respectively. The density optimized TFTs exhibited excellent performance figures with charge carrier mobilities up to 100 cm(2) V(-1) s(-1) and ON/OFF current ratios exceeding 1 × 10(6), combined with high uniformity and more than 99% of devices working as theoretically expected.

Influence of the diameter of single-walled carbon nanotube bundles on the optoelectronic performance of dry-deposited thin films.

The optoelectronic performance of thin films of single-walled carbon nanotubes (SWCNTs) was studied with respect to the properties of both individual nanotubes and their bundles. The SWCNTs were synthesized in a hot wire generator aerosol reactor, collected by gas filtration and dry-transferred onto various substrates. By thus completely avoiding liquid dispersion steps, we were able to avoid any artifacts from residual surfactants or sonication. We found that bundle lengths determined the thin-film performance, as would be expected for highly resistive bundle-bundle junctions. However, we found no evidence that contact resistances were affected by the bundle diameters, although they did play a secondary role by simply affecting the absorption. The individual SWCNT diameters and their graphitization level as gauged by the Raman D band intensity did not show any clear correlation with the overall performance.

Synthesis of ZnO tetrapods for flexible and transparent UV sensors.

ZnO tetrapods (ZnO-Ts) were synthesized in a vertical flow reactor by gas phase oxidation of Zn vapor in an air atmosphere. The morphology of the product was varied from nearly spherical nanoparticles to ZnO-Ts, together with the partial pressure of Zn and reaction temperature. MgO introduced during synthesis, increased the band gap, the optical transparency in the visible range, and also changed the ZnO-T structure. Fabricated flexible transparent UV sensors showed a 45-fold current increase under UV irradiation with an intensity of 30 µW cm(-2) at a wavelength of 365 nm and response time of 0.9 s.

Multifunctional free-standing single-walled carbon nanotube films.

We report a simple and rapid method to prepare multifunctional free-standing single-walled carbon nanotube (SWCNT) films with variable thicknesses ranging from a submonolayer to a few micrometers having outstanding properties for a broad range of exceptionally performing devices. We have fabricated state-of-the-art key components from the same single component multifunctional SWCNT material for several high-impact application areas: high efficiency nanoparticle filters with a figure of merit of 147 Pa(-1), transparent and conductive electrodes with a sheet resistance of 84 Ω/◻ and a transmittance of 90%, electrochemical sensors with extremely low detection limits below 100 nM, and polymer-free saturable absorbers for ultrafast femtosecond lasers. Furthermore, the films are demonstrated as the main components in gas flowmeters, gas heaters, and transparent thermoacoustic loudspeakers.