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Transition metal dichalcogenide (TMD) quantum dots (QDs) with defects have attracted interesting chemistry due to the contribution of vacancies to their unique optical, physical, catalytic, and electrical properties. Engineering defined defects into molybdenum sulfide (MoS

Control of materials properties has been the driving force of modern technologies. So far, materials properties have been modulated by their composition, structure, and size. Here, by using cathodoluminescence in a scanning transmission electron microscope, we show that the optical properties of stacked,100 nm thick hexagonal boron nitride (hBN) films can be continuously tuned by their relative twist angles. Due to the formation of a moiré superlattice between the two interface layers of the twisted films, a new moiré sub-band gap is formed with continuously decreasing magnitude as a function of the twist angle, resulting in tunable luminescence wavelength and intensity increase of40×. Our results demonstrate that moiré phenomena extend beyond monolayer-based systems and can be preserved in a technologically relevant, bulklike material at room temperature, dominating optical properties of hBN films for applications in medicine, environmental, or information technologies.

Two-dimensional layered materials (2D LMs) are taking the scientific world by storm. Graphene epitomizes 2D LMs with many interesting properties and corresponding applications. Following the footsteps of graphene, many other types of 2D LMs such as transition metal dichalcogenides, black phosphorus, and graphitic-phase C3N4 nanosheets are emerging to be equally interesting as graphene and its derivatives. Some of these applications such as nanomedicine do have a high probability of human exposure. This review focuses on the biological and toxicity effects of 2D LMs and their associated mechanisms linking their chemistries to their biological end points. This review aims to help researchers to predict and mitigate any toxic effects. With understanding, redesign of newer and safer 2D LMs becomes possible.

Oxidation is a scalable process for introducing nanopores in two-dimensional transitional metal dichalcogenides (TMDs) for membrane applications. The nanopore density is determined by the areal density of their nucleation sites; understanding the nature of the defects and their control would enable tailoring of TMD membranes for targeted applications. In this work, we show that the nanopore distribution is dramatically different in strained and unstrained MoS2 crystals. We correlate this spatial distribution to the underlying arrangement of dislocations in MoS2 crystals, in contrast to previously suggested sulfur vacancies. To control the nucleation density of MoS2 nanopores, we demonstrate that the pore density can be modulated by electron beam exposure prior to the nanopore formation. Raman analysis of electron beam-exposed samples indicates that hydrocarbon adsorption activates defect species other than dislocations, which significantly enhances the nanopore density in MoS2.

Two-dimensional layered molybdenum disulfide (MoS2) is a promising catalyst for hydrogen evolution reaction (HER), and a good replacement for platinum (Pt) in elelctrochemical water splitting. Most transition metal dichalcogenides (TMDs) show excellent catalytic activity, which stems from their active sites located along the edges. However, small density of the active sites in the basal plane, largely limits TMDs performance. To enhance the HER catalysis activity of MoS2, we developed an efficient and scalable approach to significantly increase the overall electrochemically active sites using mild sodium hypochlorite (NaClO) solution anisotropic etching. The effect is further enhanced by oxygen-plasma pretreatment of the material, which - upon chemical etching - leads to highly porous and highly reactive structure. The resulting chemically activated MoS2 (ca-MoS2) was systematically characterized and optimized. The optimized ca-MoS2 powder exhibits enhanced HER performance with an overpotential of 0.34 V at a current density of 0.5 mA cm−2 in this experiment due to the increasing active sites, and the Tafel slope also smaller than other samples. This chemical etching method provides new ways to design atomic structure modification, including controlling layered TMD electrochemical property and new type of transistor fabrication.

Two-dimensional transition metal dichalcogenide (TMD) materials, albeit promising candidates for applications in electronics and optoelectronics1-3, are still limited by their low electrical mobility under ambient conditions. Efforts to improve device performance through a variety of routes, such as modification of contact metals4 and gate dielectrics5-9 or encapsulation in hexagonal boron nitride10, have yielded limited success at room temperature. Here, we report a large increase in the performance of TMD field-effect transistors operating under ambient conditions, achieved by engineering the substrate's surface morphology. For MoS2 transistors fabricated on crested substrates, we observed an almost two orders of magnitude increase in carrier mobility compared to standard devices, as well as very high saturation currents. The mechanical strain in TMDs has been predicted to boost carrier mobility11, and has been shown to influence the local bandgap12,13 and quantum emission properties14 of TMDs. With comprehensive investigation of different dielectric environments and morphologies, we demonstrate that the substrate's increased corrugation, with its resulting strain field, is the dominant factor driving performance enhancement. This strategy is universally valid for other semiconducting TMD materials, either p-doped or n-doped, opening them up for applications in heterogeneous integrated electronics.

Knots in long DNA molecules are prevalent in biological systems and serve as a model system for investigating static and dynamic properties of biopolymers. We explore the dynamics of knots in double-stranded DNA in a new regime of nanometer-scale confinement, large forces, and short time scales, using solid-state nanopores. We show that DNA knots undergo isomorphic translocation through a nanopore, retaining their equilibrium morphology by swiftly compressing in a lateral direction to fit the constriction. We observe no evidence of knot tightening or jamming, even for single-digit nanopores. We explain the observations as the malleability of DNA, characterized by sharp buckling of the DNA in nanopores, driven by the transient disruption of base pairing. Our molecular dynamics simulations support the model. These results are relevant not only for the understanding of DNA packing and manipulation in living cells but also for the polymer physics of DNA and the development of nanopore-based sequencing technologies.

Graphene, an atomic thin sheet of monolayer carbon atoms, is deemed to replace silicon and revolutionize the electronics industry. It has massless Dirac fermions and exhibits ballistic charge transport, a prerequisite for upcoming futuristic nanodevices, such as field-effect transistors, sensors, etc. However, it lacks an intrinsic electronic bandgap and thus is obsolete for use in these applications. In this work, a first principles study is used to predict the opening of a bandgap in graphene by an engineered introduction of modulations in its lattice. Moreover, it is found that the atomic modulation and the addition of hydrogen atoms at sub-lattices induce magnetism in the graphene sheets. The hydrogen-induced itinerant magnetism (ferromagnetic or antiferromagnetic) depends on the type of defects and the structure of the sheet. This hydrogen-induced spin promotes the role of graphene as a potential material for magnetic memory device applications. Furthermore, with the creation of atomic vacancies and quasilocalized states, it is deemed to allow selective permeation (water/gas) and thus paves the way for its use in-filtration (membrane technology) and hydrogen storage applications.

Layer-by-layer (LbL) self-assembled stacked Testudo-like MoS2 superstructures carrying cancer drugs are formed from nanosheets controllably assembled with sequence-based DNA oligonucleotides. These superstructures can disassemble autonomously in response to cancer cells’ heightened ATP metabolism. First, we functionalize MoS2 nanosheets (MoS2-NS) nanostructures with DNA oligonucleotides having thiol-terminated groups (DNA/MoS2-NS) via strong binding to sulfur atom defect vacancies on MoS2 surfaces. The driving force to assemble into a higher-order DNA/MoS2-NS superstructure is guided by a linker aptamer that induced interlayer assembly. In the presence of target ATP molecules, these multilayer superstructures disassembled as a consequence of stronger binding of ATP molecules with the linking aptamers. This design plays a dual role of protection and delivery by LbL stacked MoS2-NS similar in concept to a Greek Testudo. These superstructures present a protective armor-like shell of MoS2-NS, which still rema...

Nanostructured graphene-oxide (GO) laminate membranes, exhibiting ultrahigh water flux, are excellent candidates for next generation nanofiltration and desalination membranes, provided the ionic rejection could be further increased without compromising the water flux. Using microscopic drift-diffusion experiments, we demonstrated the ultrahigh charge selectivity for GO membranes, with more than order of magnitude difference in the permeabilities of cationic and anionic species of equivalent hydration radii. Measuring diffusion of a wide range of ions of different size and charge, we were able to clearly disentangle different physical mechanisms contributing to the ionic sieving in GO membranes: electrostatic repulsion between ions and charged chemical groups; and the compression of the ionic hydration shell within the membrane's nanochannels, following the activated behavior. The charge-selectivity allows us to rationally design membranes with increased ionic rejection and opens up the field of ion exchange and electrodialysis to the GO membranes.

Oxidation is a scalable process for introducing nanopores in two-dimensional transitional metal dichalcogenides (TMDs) for membrane applications. The nanopore density is determined by the areal density of their nucleation sites; understanding the nature of the defects and their control would enable tailoring of TMD membranes for targeted applications. In this work, we show that the nanopore distribution is dramatically different in strained and unstrained MoS

Nanometre-scale graphitic capillaries with atomically flat walls are engineered and studied, revealing unexpectedly fast transport of liquid water through channels that accommodate only a few layers of water. Artificial nanometre-sized capillaries have enabled new research and led to the emergence of nanofluidics, but surface roughness in particular makes it very challenging to exactly control their dimensions. Andre Geim and colleagues now show that van der Waals assembly can produce narrow and smooth capillaries that have atomically flat top and bottom graphite sheets, separated by spacers made from a precisely controlled number of graphene layers. Water transport through the channels, which range in height from a single atomic plane to dozens of them, is unexpectedly fast and speeds up further in channels that accommodate only a few layers of water. The fabrication method is expected to give access to a wide range of capillaries with atomically precise sizes, and with permeation properties that are tunable by the choice of two-dimensional material used for creating the channel walls. Nanometre-scale pores and capillaries have long been studied because of their importance in many natural phenomena and their use in numerous applications1. A more recent development is the ability to fabricate artificial capillaries with nanometre dimensions, which has enabled new research on molecular transport and led to the emergence of nanofluidics2,3,4. But surface roughness in particular makes it challenging to produce capillaries with precisely controlled dimensions at this spatial scale. Here we report the fabrication of narrow and smooth capillaries through van der Waals assembly5, with atomically flat sheets at the top and bottom separated by spacers made of two-dimensional crystals6 with a precisely controlled number of layers. We use graphene and its multilayers as archetypal two-dimensional materials to demonstrate this technology, which produces structures that can be viewed as if individual atomic planes had been removed from a bulk crystal to leave behind flat voids of a height chosen with atomic-scale precision. Water transport through the channels, ranging in height from one to several dozen atomic planes, is characterized by unexpectedly fast flow (up to 1 metre per second) that we attribute to high capillary pressures (about 1,000 bar) and large slip lengths. For channels that accommodate only a few layers of water, the flow exhibits a marked enhancement that we associate with an increased structural order in nanoconfined water. Our work opens up an avenue to making capillaries and cavities with sizes tunable to angstrom precision, and with permeation properties further controlled through a wide choice of atomically flat materials available for channel walls.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) nanostructures have been widely applied in environmental and biological analysis, biomedicine, electronic devices, and hydrogen evolution catalysis. Meanwhile, this excitement in 2D TMDs has spilled over to their counterparts of different dimensionalities like one-dimensional (1D) and zero-dimensional (0D) TMDs nanostructures. Eventual physical and chemical properties of TMDs nanostructures still remain to be highly dependent on their dimensionalities and size scale, and recently creatively exploring these physical and chemical properties is extremely impactful for the sensing field of TMD nanomaterials. Herein, we review a wide range of sensing applications based on not only graphene-like 2D TMDs nanostructures but also the rapidly emerging subclasses of 1D, and 0D TMDs nanostructures. Their unique and interesting structures, excellent properties, and valid preparation methods are also included and the analytical objectives, ranging from heavy metal ions to small molecules, from DNA to proteins, from liquids to even vapors, can be met with extremely high selectivity and sensitivity. We have also analyzed our current understanding of 0D and 1D TMDs nanostructures and learning from graphene with the goal of contributing fresh ideas to the overall development of more advanced future TMDs based sensors.

An ion-beam exposure can be applied to form defects in supported graphene which can be tuned with energies and fluence. However, recent results show that nuclear energy loss process is the dominant mechanism in defect formation. In the present work, it is shown that in low energy regime (1–10 keV Ar+ ions), both nuclear and electronic energy losses play an important role in forming different types of defects. Here we show a linear relation between defect induction efficiency and energy loss. Conductive atomic force microscopy shows a significant reduction in the current through supported graphene after ion beam exposure.

Two-dimensional (2D) transition metal dichalcogenides can be alloyed by substitution at the metal atom site with negligible effect on lattice strain, but with significant influence on optical and electrical properties. In this work, we establish the relationship between composition and optical properties of the MoxW1−xS2 alloy by investigating the effect of continuously-varying composition on photoluminescence intensity. We developed a new process for growth of two-dimensional MoxW1−xS2 alloys that span nearly the full composition range along a single crystal, thus avoiding any sample-related heterogeneities. The graded alloy crystals were grown using a diffusion-based chemical vapor deposition (CVD) method that starts by synthesizing a WS2 crystal with a graded point defect distribution, followed by Mo alloying in the second stage. We show that point defects promote the diffusion and alloying, as confirmed by Raman and photoluminescence measurements, density functional theory calculations of the reaction path, and observation that no alloying occurs in CVD-treated exfoliated crystals with low defect density. We observe a significant dependence of the optical quantum yield as a function of the alloy composition reaching the maximum intensity for the equicompositional Mo0.5W0.5S2 alloy. Furthermore, we map the growth-induced strain distribution within the alloyed crystals to decouple composition and strain effects on optical properties: at the same composition, we observe significant decrease in quantum yield with induced strain. Our approach is generally applicable to other 2D materials as well as the optimization of other composition-dependent properties within a single crystal.

Squeezing through a hole Transport of an ion is usually directly related to its hydrated radius and assumed to be nonflexible. Either a hydrated ion fits through an aperture or it does not, and shape should play a dominant role rather than charge. Esfandiar et al. created nanofluidic devices by stacking structured bulk materials, including graphite, boron nitride, and molybdenum disulfide. They investigated the transport of ions in aqueous solutions through the nanochannels in the devices. Unexpectedly, they observed different behavior for ions of similar hydrated size but opposite charge. Science , this issue p. 511

Layer-by-layer (LbL) self-assembled stacked Testudo-like MoS

Isolated, atomically thin conducting membranes of graphite, called graphene, have recently been the subject of intense research with the hope that practical applications in fields ranging from electronics to energy science will emerge. The atomic thinness, stability and electrical sensitivity of graphene motivated us to investigate the potential use of graphene membranes and graphene nanopores to characterize single molecules of DNA in ionic solution. Here we show that when immersed in an ionic solution, a layer of graphene becomes a new electrochemical structure that we call a trans-electrode. The trans-electrode’s unique properties are the consequence of the atomic-scale proximity of its two opposing liquid–solid interfaces together with graphene’s well known in-plane conductivity. We show that several trans-electrode properties are revealed by ionic conductance measurements on a graphene membrane that separates two aqueous ionic solutions. Although our membranes are only one to two atomic layers thick, we find they are remarkable ionic insulators with a very small stable conductance that depends on the ion species in solution. Electrical measurements on graphene membranes in which a single nanopore has been drilled show that the membrane’s effective insulating thickness is less than one nanometre. This small effective thickness makes graphene an ideal substrate for very high resolution, high throughput nanopore-based single-molecule detectors. The sensitivity of graphene’s in-plane electronic conductivity to its immediate surface environment and trans-membrane solution potentials will offer new insights into atomic surface processes and sensor development opportunities.

We present the ESR system with dielectric resonators for performing the high-pressure measurements. This system is constructed by combining the diamond anvil cell (or sapphire anvil cell) with the double-stacked dielectric resonators, which are commercially available. The maximum pressure is obtained to be about 15 kbar and this high-pressure ESR system holds its availability from room temperature to Liq. He temperature. This system has a potential to perform ESR measurements under high-pressure with high accuracy. As an example, we show its application to the polymerization in the TDAE–C 60 organic ferromagnet under high pressure.

Through 13C NMR spin lattice relaxation ( T1) measurements in cubic Na2C60, we detect a gap in its electronic excitations, similar to that observed in tetragonal A4C60. This establishes that Jahn-Teller distortions (JTD) and strong electronic correlations must be considered to understand the behavior of even electron systems, regardless of the structure. Furthermore, in metallic Na2CsC60, a similar contribution to T1 is also detected for 13C and 133Cs NMR, implying the occurrence of excitations typical of JT distorted C( 2-)60 (or equivalently C( 4-)60). This supports the idea that dynamic JTD can induce attractive electronic interactions in odd electron systems.

Solid solutions of C 59 N azafullerene in C 60 with concentrations of 10 −5 to 10 −4 were produced in large quantities in an electric gas discharge tube. C 59 N is a stable monomeric substituent molecule in crystalline C 60 . The isotropic 14 N and 13 C hyperfine coupling constants measured by electron spin resonance (ESR) are characteristic of the extent of delocalization of the charge over the cage and are a sensitive test of electronic structure calculations. The C 59 N reorientational activation energy measured below the face centered cubic (fcc) to simple cubic (sc) transition is 2300 K. This value is similar to that of the matrix C 60 molecules, indicating that C 59 N–C 60 intermolecular interactions are weak.

The magnetic properties of carbon nanohorns were investigated by electron spin resonance. Two decoupled electronic systems were found. The first one, which has a spin susceptibility linearly varying with temperature, was tentatively attributed to two-dimensional graphenelike sheets. The second contribution, showing a Curie-like behavior with a non-negligible Pauli part, was ascribed to the interior of the structures of coagulated nanohorns. Surprisingly, this second line shows a strong suppression of susceptibility at 17 K. Due to a concomitant line broadening and g shift below 17 K, this transition was assigned to antiferromagnetic pairing of localized spins. The low spin concentration is implying that a kind of magnetism, mediated by the free spin carriers, has to be invoked to explain the phenomenon.

It has recently been recognized that solid-state nanopores in single-atomic-layer graphene membranes can be used to electronically detect and characterize single long charged polymer molecules. We have now fabricated nanopores in single-layer graphene that are closely matched to the diameter of a double-stranded DNA molecule. Ionic current signals during electrophoretically driven translocation of DNA through these nanopores were experimentally explored and theoretically modeled. Our experiments show that these nanopores have unusually high sensitivity (0.65 nA/A) to extremely small changes in the translocating molecule’s outer diameter. Such atomically short graphene nanopores can also resolve nanoscale-spaced molecular structures along the length of a polymer, but they do so with greatest sensitivity only when the pore and molecule diameters are closely matched. Modeling confirms that our most closely matched pores have an inherent resolution of ≤0.6 nm along the length of the molecule.

A fabrication method for positioning and embedding a single-walled carbon nanotube (SWNT) across the diameter of a solid state nanopore is presented. Chemical vapor deposition (CVD) is used to grow SWNTs over arrays of focused ion beam (FIB) milled pores in a thin silicon nitride membrane. This typically yields at least one pore whose diameter is centrally crossed by a SWNT. The final diameter of the FIB pore is adjusted to create a nanopore of any desired diameter by atomic layer deposition (ALD), simultaneously embedding and insulating the SWNT everywhere but in the region that crosses the diameter of the final nanopore, where it remains pristine and bare. This nanotube-articulated nanopore is an important step towards the realization of a new type of detector for biomolecule sensing and electronic characterization, including DNA sequencing.

We identify a contribution to the ionic current noise spectrum in solid-state nanopores that exceeds all other noise sources in the frequency band 0.1–10 kHz. Experimental studies of the dependence of this excess noise on pH and electrolyte concentration indicate that the noise arises from surface charge fluctuations. A quantitative model based on surface functional group protonization predicts the observed behaviors and allows us to locally measure protonization reaction rates. This noise can be minimized by operating the nanopore at a deliberately chosen pH.

Thermal homolysis of biazafullerenyl, (C59N)2, has been studied by CW ESR in the temperature range 290−829 K. The room-temperature ESR spectrum consists of a single sharp narrow line whose intensity is consistent with the presence of a low concentration of defect spins (1 spin per 600 C59N units). Around 500 K, three sharp lines emerge on top of the impurity signal and are assigned to the 14N(I = 1) hyperfine spectrum with a hyperfine coupling constant, aiso = 3.60(1) G and a g-factor of 2.0004(2). The intensity of the observed 14N hyperfine spectrum directly measures the fraction of the C59N• radicals formed as the temperature increases, growing from ∼50 ppm at 516 K to ∼300 ppm at 740 K. The temperature evolution of this fraction allows us to extract an estimate of the binding energy as 7(1) kcal/mol. The high temperature treatment of the sample eventually results in the partial decomposition and formation of C60.

The temperature dependence of the electron spin relaxation time in MgB2 is anomalous as it does not follow the temperature dependence of the resistivity above 150 K, it has a maximum around 400 K, and it decreases for higher temperatures. This violates the well established Elliot-Yafet theory of electron spin relaxation in metals. We show that the anomaly occurs when the quasi-particle scattering rate (in energy units) becomes comparable to the energy difference between the conduction- and a neighboring band. We find that the anomalous behavior is related to the unique band structure of MgB$_2$ and the large electron-phonon coupling. The saturating spin-lattice relaxation can be regarded as the spin transport analogue of the Ioffe-Regel criterion of electron transport.

A nanopore-based device provides single-molecule detection and analytical capabilities that are achieved by electrophoretically driving molecules in solution through a nano-scale pore. The nanopore provides a highly confined space within which single nucleic acid polymers can be analyzed at high throughput by one of a variety of means, and the perfect processivity that can be enforced in a narrow pore ensures that the native order of the nucleobases in a polynucleotide is reflected in the sequence of signals that is detected. Kilobase length polymers (single-stranded genomic DNA or RNA) or small molecules (e.g., nucleosides) can be identified and characterized without amplification or labeling, a unique analytical capability that makes inexpensive, rapid DNA sequencing a possibility. Further research and development to overcome current challenges to nanopore identification of each successive nucleotide in a DNA strand offers the prospect of 'third generation' instruments that will sequence a diploid mammalian genome for approximately $1,000 in approximately 24 h.

The 23Na nuclear magnetic resonance (NMR) spectrum, spin-lattice and spin– spin relaxation, as well as spin echo double resonance (SEDOR) are investigated in the ternary alkali fulleride compound Na2CsC60 in the temperature range of 10– 300 K. The NMR line associated with the tetrahedral sodium site is split below 170K (T and T lines) similarly to Rb3C60 although the crystal structures of these two materials are different. SEDOR measurements prove that the T and T sites are microscopically close. The merger of the two lines at about 170K is attributed to motional narrowing resulting from a site exchange due to angular reorientations of the C60 molecules. The exchange dynamics inferred from the spectra, spin– spin relaxation, and spin-lattice relaxation are all consistent and agree with inelastic neutron scattering, supporting our proposal that the observed T-T splitting originates from different local fullerene configurations around the tetrahedral alkaline sites.

We have investigated sodium containing fullerene superconductors ${\mathrm{Na}}_{2}A{\mathrm{C}}_{60}$, $A=\mathrm{Cs}$, Rb, and K, by $^{23}\mathrm{Na}$ nuclear magnetic resonance (NMR) spectroscopy at $7.5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ in the temperature range of $10\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Despite the structural differences from the ${\mathrm{Rb}}_{3}{\mathrm{C}}_{60}$ class of fullerene superconductors, in these compounds the NMR line of the tetrahedrally coordinated alkali nuclei also splits into two lines ($T$ and ${T}^{\ensuremath{'}}$) at low temperature. In ${\mathrm{Na}}_{2}\mathrm{Cs}{\mathrm{C}}_{60}$ the splitting occurs at $170\phantom{\rule{0.3em}{0ex}}\mathrm{K}$; in the quenched cubic phase of ${\mathrm{Na}}_{2}\mathrm{Rb}{\mathrm{C}}_{60}$ and ${\mathrm{Na}}_{2}\mathrm{K}{\mathrm{C}}_{60}$ we observe split lines at $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. By detailed investigations of the spectrum, spin-spin, and spin-lattice relaxations as well as spin-echo double resonance in ${\mathrm{Na}}_{2}\mathrm{Cs}{\mathrm{C}}_{60}$ we show that these two different tetrahedral sites are mixed on a microscopic scale. The $T$ and ${T}^{\ensuremath{'}}$ sites differ in the orientation of first-neighbor ${\mathrm{C}}_{60}$ molecules. We present evidence that the orientations of neighboring molecules are uncorrelated. Thermally activated molecular reorientations cause an exchange between the $T$ and ${T}^{\ensuremath{'}}$ sites and motional narrowing at high temperature. We infer the same activation energy $3300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the temperature range $125\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The spin-lattice relaxation rate is the same for $T$ and ${T}^{\ensuremath{'}}$ down to $125\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ but different below. Both the spin-lattice relaxation rate and Knight shift are strongly temperature dependent in the whole range investigated. We interpret this temperature variation by the effect of phonon excitations involving the rigid librational motion of the ${\mathrm{C}}_{60}$ molecules. By extending the understanding of the structure and molecular dynamics of ${\mathrm{C}}_{60}$ superconductors, these results may help in clarifying the effects of the structure on the superconducting properties.

We present a newly developed microwave probe head that accommodates a gasketed sapphire anvil cell (SAC) for performing sensitive electron spin resonance (ESR) measurements under high-hydrostatic pressures. The system was designed around commercially available dielectric resonators (DRs) having the dielectric permittivity of approximately 30. The microwave resonant structure operates in a wide-stretched double-stacked geometry and resonates in the lowest cylindrical quasi TE(011) mode around 9.2 GHz. The most vital parts of the probe's microwave heart were made of plastic materials, thus making the resonant structure transparent to magnetic field modulation at 100 kHz. The overall ESR sensitivity of the probe was demonstrated for a small speck of 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) positioned in the gasket of the SAC, using water as the pressure-transmitting medium. The system was also used for studying pressure-induced changes in spin-relaxation mechanisms of a quasi-1D-conducting polymer, K(1)C(60). For small samples located in the sample hole of the gasket the probe reveals sensitivity that is only approximately 3 times less than that yielded by regular ESR cavities.

It is now generally accepted that MgB2 is a phononmediated superconductor 1–3 which owes its unusual properties to the strong differences in the electron-phonon coupling of its disconnected Fermi surface sheets. 4 The two-gap model assigns a large superconductor gap to the cylindrical Fermi surface sheets originating from B – B bonds and a much smaller gap to the sheets of electrons. The model successfully describes the temperature dependence of several properties like the specific heat 5 or the electron tunneling spectra 6 in zero magnetic field while predictions for finite magnetic fields are less tested. The strong electronphonon coupling on the sheets maintains superconductivity to about Hc2 c 2.5 T for fields perpendicular to and H c2 ab 16 T for fields lying in the crystalline a ,b boron planes. 7 It has been suggested that the weaker gap is closed by much smaller fields H c2 without destroying superconductivity. The actual value of Hc2 is rather uncertain: from single-crystal specific heat experiments 8 an isotropic Hc2 = 0.3– 0.4 T, from vortex imaging 9 0.13 T has been estimated, while point contact tunneling experiments 10–13 suggest values between 1 and 2 T.

Electron spin resonance and ab initio electronic structure calculations show an intricate relation between molecular rotation and chemical bonding in the dilute solid solution. The unpaired electron of C59N is delocalized over several C60 molecules above 700 K, while at lower temperatures it remains localized within short range. The data suggest that below 350 K rigid C59N-C60 heterodimers are formed in thermodynamic equilibrium with dissociated rotating molecules. The structural fluctuations between heterodimers and dissociated molecules are accompanied by simultaneous electron spin transfer between C60 and C59N molecules. The calculation confirms that in the C59N-C60 heterodimer the spin density resides mostly on the C60 moiety, while it is almost entirely on C59N in the dissociated case.

Electron spin resonance and ab initio electronic structure calculations show an intricate relation between molecular rotation and chemical bonding in the dilute solid solution C59N:C60. The unpaired electron of C59N is delocalized over several C60 molecules above 700 K, while at lower temperatures it remains localized within short range. The data suggest that below 350 K rigid C59N‐C60 heterodimers are formed in thermodynamic equilibrium with dissociated rotating molecules. The structural fluctuations between heterodimers and dissociated molecules are accompanied by simultaneous electron spin transfer between C60 and C59N molecules. The calculation confirms that in the C59N‐C60 heterodimer the spin density resides mostly on the C60 moiety, while it is almost entirely on C59N in the dissociated case.

We report the realization of nanotube-based quantum dot structures that use local electrostatic gating to produce individually controllable dots in series along a nanotube. Electrostatic top-gates produce depletion regions in the underlying tube; a pair of such depletion regions defines a quantum dot. Transparencies of tunnel barriers as well as the electrostatic energies, within single and multiple dots, can be tuned by gate voltages. The approach allows accurate control over multiple devices on a single tube, and serves as a design paradigm for nanotube-based electronics and quantum systems., Comment: related papers at http://marcuslab.harvard.edu

High-pressure electron spin resonance (ESR) measurements were performed on tetrakis(dimethylamino) ethylene (TDAE)-C-60 single crystals and stability of the polymeric phase was established in the P-T parameter space. At 7 kbar the system undergoes a ferromagnetic to paramagnetic phase transition due to the pressure-induced polymerization. The polymeric phase remains stable after the pressure release. The depolymerization of the pressure-induced phase was observed at a temperature of 520 K, revealing an unexpectedly high thermal stability of the polymer. Below room temperature, the polymeric phase behaves as a simple Curie-type insulator with one unpaired electron spin per chemical formula. The TDAE(+) donor-related unpaired electron spins, formerly ESR silent, become active above a temperature of 320 K, which demonstrates that the magnetic properties are profoundly defined by miniscule reorientation of TDAE molecules.

We present a nuclear magnetic resonance (NMR) study of ${\mathrm{Na}}_{2}{\mathrm{C}}_{60}$ and ${\mathrm{K}}_{4}{\mathrm{C}}_{60},$ two compounds that are related by electron-hole symmetry in the ${\mathrm{C}}_{60}$ triply degenerate conduction band. In both systems, it is known that NMR spin-lattice relaxation rate ${(1/T}_{1})$ measurements detect a gap in the electronic structure, most likely related to singlet-triplet excitations of the Jahn-Teller distorted (JTD) ${\mathrm{C}}_{60}^{2\ensuremath{-}}$ or ${\mathrm{C}}_{60}^{4\ensuremath{-}}.$ However, the extended temperature range of the measurements presented here (10 K to 700 K) allows one to reveal deviations with respect to this general trend, both at high and low temperatures. Above room temperature, ${1/T}_{1}$ deviates from the activated law that one would expect from the presence of the gap and saturates. In the same temperature range, a lowering of symmetry is detected by the appearance of quadrupole effects on the ${}^{23}\mathrm{Na}$ spectra of ${\mathrm{Na}}_{2}{\mathrm{C}}_{60}.$ In ${\mathrm{K}}_{4}{\mathrm{C}}_{60},$ modifications of the ${}^{13}\mathrm{C}$ spectra line shapes also indicate a structural modification. We discuss this high-temperature deviation in terms of a coupling between JTD and local symmetry. At low temperatures, ${1/T}_{1}T$ tends to a constant value for ${\mathrm{Na}}_{2}{\mathrm{C}}_{60},$ both for ${}^{13}\mathrm{C}$ and ${}^{23}\mathrm{Na}$ NMR. This indicates a residual metallic character, which emphasizes the proximity of metallic and insulting behaviors in alkali fullerides.

We present 13C NMR spin-lattice relaxation measurements (1/T1) in Na2CsC60 and Rb3C60 from 10 to 700K. The large temperature range of this measurement allow to define unambiguously an increase of 1/T1T with increasing temperature, which is anomalous in a simple metallic picture, where the Korringa law predicts 1/T1T = cst. We attribute this increase to the existence of an additional relaxation channel related to singlet-triplet (ST) excitations of Jahn-Teller distorted C60^{2-} and C60^{4-}. These units are formed within the metal on very short time scales (10^{-14} sec) that do not imply static charge segregation. We show that the amplitude of the ST component depends directly on the density of states, which indicates an interplay between metallic and molecular excitations. Such an interaction is also revealed by the high temperature behavior of Na2CsC60 and CsC60, that we then discuss. A divergence between the behaviors of 1/T1, the NMR shift and the ESR susceptibility is interpreted as the result of a rapid increase of the lifetime of the charge carriers, signaling a tendency to charge localization. In our analysis, the particular stability of C60^{2n-} is then a common feature of all known metallic fullerides and allow to reconcile apparently contradicting properties of these systems., 10 pages, 7 figures