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We use the weighted histogram analysis method [S. Kumar, D. Bouzida, R. H. Swendsen, P. A. Kollman, and J. M. Rosenberg, J. Comput. Chem. 13, 1011 (1992)] to calculate the free energy surface of the ST2 model of water as a function of density and bond-orientational order. We perform our calculations at deeply supercooled conditions (T = 228.6 K, P = 2.2 kbar; T = 235 K, P = 2.2 kbar) and focus our attention on the region of bond-orientational order that is relevant to disordered phases. We find a first-order transition between a low-density liquid (LDL, ρ ≈ 0.9 g/cc) and a high-density liquid (HDL, ρ ≈ 1.15 g/cc), confirming our earlier sampling of the free energy surface of this model as a function of density [Y. Liu, A. Z. Panagiotopoulos, and P. G. Debenedetti, J. Chem. Phys. 131, 104508 (2009)]. We demonstrate the disappearance of the LDL basin at high pressure and of the HDL basin at low pressure, in agreement with independent simulations of the system's equation of state. Consistency between directly computed and reweighted free energies, as well as between free energy surfaces computed using different thermodynamic starting conditions, confirms proper equilibrium sampling. Diffusion and structural relaxation calculations demonstrate that equilibration of the LDL phase, which exhibits slow dynamics, is attained in the course of the simulations. Repeated flipping between the LDL and HDL phases in the course of long molecular dynamics runs provides further evidence of a phase transition. We use the Ewald summation with vacuum boundary conditions to calculate long-ranged Coulombic interactions and show that conducting boundary conditions lead to unphysical behavior at low temperatures.

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238 wileyonlinelibrary.com COMMUNICATION available highly purifi ed CNTs, with reported purity of up to 99.9% or even higher. [ 22 ] Another factor leads to the slow progress on the development of high-performance CNT IR detectors is that device structure and operation mode have not been optimized for solution-processed CNTs to achieve performance comparable with that of state-of-the-art commercial IR detectors. [ 23,24 ] In this letter, we show that high-performance photodiodes can be constructed using solution-processed CNTs via a dopingfree technique. In contrast to other photodetectors that use photocurrent as the signal, [ 8,[11][12][13][14][15][16][17][18][19][20] here we exploit using photovoltage as the signal. The major benefi t of using photovoltage is that the commonly occurring shot noise and 1/ f noise can be signifi cantly suppressed. In addition, signal can be multiplied via introducing virtual contacts, which leads to further improvement on signal-to-noise ratio. A prototype CNT IR detector is demonstrated, which works at room temperature and shows broadband response, high responsivity and detectivity that are comparable to that of state-of-the-art room temperature semiconductor IR detectors. It is also demonstrated that our CNT IR detectors have excellent stability, as a result of the dopingfabrication process used here, with time, under high power illumination and at rigorous temperature conditions. An array of 150 × 150 photodetectors on a single chip is fabricated, with tested yield of 100% and high device uniformity, showing the potential for large-scale fabrication capability and imager applications.In a typical photovoltaic device, a built-in fi eld is essential for the effi cient separation of photoinduced electron-hole pairs. For CNT-based diodes, ideal rectifi cation behavior has been realized by using split gates or asymmetric contacts on individual CNTs. [ 9,10,13 ] However, light absorption in these devices is usually very weak. It is thus advantageous to construct a photodiode using CNT fi lm with more CNTs in the device channel. However, solution-processed CNT-fi lm-based diodes showing excellent rectifi cation effect have not been realized. [ 23,24 ] Here, we show that such high-performance diode based on solutionprocessed CNT fi lm can be realized by using a doping-free technique in a barrier-free-bipolar diode (BFBD) device geometry as depicted in Figure 1 a. In this device geometry, Sc and Pd are asymmetrically contacted to a CNT fi lm made by a liquidphase deposition technique on an n + silicon/SiO 2 substrate (see Figure S1a,b, Supporting Information). Thus, p-region and n-region are automatically formed adjacent to the contacts by charge transfer from the contacts. This process involves no intentionally introduced dopants, no extra defects on CNTs,

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