Magnetizations and magnetic moments of free cobalt clusters Co(N) (12 < N < 200) in a cryogenic (25 K < or = T < or = 100 K) molecular beam were determined from Stern-Gerlach deflections. All clusters preferentially deflect in the direction of the increasing field and the average magnetization resembles the Langevin function for all cluster sizes even at low temperatures. We demonstrate in the avoided crossing model that the average magnetization may result from adiabatic processes of rotating and vibrating clusters in the magnetic field and that spin relaxation is not involved. This resolves a long-standing problem in the interpretation of cluster beam deflection experiments with implications for nanomagnetic systems in general.

Electric deflections of gas-phase, cryogenically cooled, neutral niobium clusters [NbN; number of atoms (N) = 2 to 150, temperature (T) = 20to 300kelvin], measured in molecular beams, show that cold clusters may attain an anomalous component with very large electric dipole moments. In contrast, room-temperature measurements show normal metallic polarizabilities. Characteristic energies kBTG(N) [Boltzmann constant kB times a transition temperature TG(N)] are identified, below which the ferroelectric-like state develops. Generally, TG decreases [110 > TG(N) > 10K] as N increases, with pronounced even-odd alternations for N > 38. This new state of metallic matter may be related to bulk superconductivity.

The response of (H2O)(n=3-18) clusters to an electric field is studied by beam deflection. All clusters deflect uniformly, behaving as polarizable particles. The effective polarizabilities exceed the electronic component and increase as the clusters are cooled, revealing a large permanent dipole contribution. The results resolve a discrepancy concerning the polarity of water clusters and show that all species access conformations with moments exceeding 1 D. The data show no evidence for a freezing transition down to approximately 120 K, but suggest a shift in the conformer arrangement at n=8-9.

Above a critical temperature, a supported single domain ferromagnetic particle responds to an applied magnetic field as if it were a paramagnet with a very large spin. Its average magnetization is given by the Langevin equation as expected from simple thermodynamic considerations. The average magnetization of an ensemble of unsupported ferromagnetic clusters also approximately follows the Langevin equation even for small clusters in a low-temperature ensemble. The reason is not obvious because there is no heat bath for low-energy clusters so that elementary thermodynamic requirements for the Langevin equation are not satisfied. We investigated the magnetic deflections of cobalt clusters ͑Co N , 12Յ N Յ 200͒ using molecular-beam methods over a wide range of temperatures ͑20Յ T Յ 100 K͒ and magnetic fields ͑0 Յ B Յ 2 T͒. A distribution of magnetization is observed for the cluster beams. Previously, we showed that the average magnetization of the cluster beam follows Langevin function closely for all temperatures and magnetic fields investigated, and proposed an avoided-crossing model that takes into account interacting spin-rotational states. In this paper, we report a comprehensive study of the magnetization distribution and present in depth the avoided-crossing model. The model explains both the average and the width of the magnetization distributions of the cluster beam in terms of the ensemble temperature without requiring that individual clusters have defined temperatures. We also show that the spin-relaxation model is the high-temperature limit of the avoided-crossing model. Atoms ClustersIn a beam − Յ M single Յ 0 Ͻ M single Յ P͑M single ͒ = const P͑M single ͒: broad and asymmetric M =0 M / = B r ͑B / k B T͒ ⌬M = / ͱ 3 ⌬M Ͼ 0 On a substrate ͑above the blocking temperature͒ M single = B r ͑B / k B T͒ M single = B r ͑B / k B T͒ P͑M single ͒ = ␦͓M single − B r ͑B / k B T͔͒ P͑M single ͒ = ␦͓M single − B r ͑B / k B T͔͒ M / = B r ͑B / k B T͒ M / = B r ͑B / k B T͒ ⌬M =0 ⌬M =0 XU et al. PHYSICAL REVIEW B 78, 054430 ͑2008͒ 054430-2

Magnetic moments of Co(N)Mn(M) and Co(N)V(M) clusters (N < or = 60; M < or = N/3) are measured in molecular beams using the Stern-Gerlach deflection method. Surprisingly, the per atom average moments of Co(N)Mn(M) clusters are found to increase with Mn concentration, in contrast to bulk CoMn. The enhancement with Mn doping is found to be independent of cluster size and composition in the size range studied. Meanwhile, Co(N)V(M) clusters show reduction of average moments with increasing V doping, consistent with what is expected in bulk CoV. The results are discussed within the virtual bound states model.

Molecular beam Stern-Gerlach deflection measurements on Nb clusters (Nb(N), N<100) show that at very low temperatures the odd-N clusters deflect due to a single unpaired spin that is uncoupled from the cluster. At higher temperatures the spin is coupled and no deflections are observed. Spin uncoupling occurs concurrently with the transition to the recently found ferroelectric state, which has superconductor characteristics [Science 300, 1265 (2003)]]. Spin uncoupling (also seen in V, Ta, and Al clusters) is analogous to the reduction of spin-relaxation rates observed in bulk superconductors below T(c).

Nitrogen dioxide (NO 2 ) is one of the most dangerous air pollutants that can affect human health even at the ppb (part per billion) level. Thus, the superior sensing performance of nitrogen dioxide gas sensors is an imperative for real-time environmental monitoring. Traditional solid-state sensors based on metal-oxide transistors have the drawbacks of high power consumption, high operating temperature, poor selectivity, and difficult integration with other electronics. In that respect, graphene-based gas sensors have been extensively studied as potential replacements. However, their advantages of high sensing efficiency, low power consumption, and simple electronic integration have been countered by their slow response and poor repeatability. Here, we report the fabrication of high-performance ultraviolet (UV)-assisted room temperature NO 2 sensors based on chemical vapor deposition-grown graphene. UV irradiation improves the response of the sensor sevenfold with respect to the dark condition attaining 26% change in resistance at 100 ppm NO 2 concentration with a practical detection limit below 1 ppm (42.18 ppb). In addition, the recovery time was shortened fivefold to a few minutes and the excellent repeatability. This work may provide a promising and practical method to mass produce room-temperature NO 2 gas sensors for real-time environment monitoring due to its simple fabrication process, low cost, and practicality.

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