Physical properties of materials at the nanoscale

这项研究计划旨在了解材料的原子级结构如何在更大范围内影响其性能，重点是纳米级系统，即。纳米粒子、纳米结等。为此，我们求助于各种计算方案，提供系统的相关物理，特别是分子动力学模拟，同时允许我们单独检查不同参数的影响。我们的研究计划将集中在三个不同的主题上：*(I)激光与物质的相互作用，特别是超短(几飞秒到数百ps)对固体材料的烧蚀，强激光脉冲，这是我们过去在很大程度上促成的一个问题。我们将特别研究烧蚀和随后在浸入液体(通常是水)中的目标中生产纳米颗粒的问题，这是一种常规用于生产纳米颗粒的方法，这些纳米颗粒本身也被用于医疗应用。我们将研究激光脉冲的“操纵”，以优化抛射物质的结构并实现量身定做的特性--一种称为脉冲整形的方法。最后，我们将研究热致密物质和“等离子体”区域，其中物理过程由非热过程主导，特别是库仑效应(排斥、爆炸、电荷效应)。*(Ii)纳米热学：我们感兴趣的是确定热在纳米材料，特别是半导体和碳基材料中的传输和消散方式。为此，我们研究了声子对热传输的贡献，即各种模式如何贡献单个热流以及它们如何相互关联。我们发现，这些关联起到了对抗热流衰减的作用，这就解释了为什么一维材料是如此好的导热材料。我们将把这项工作扩展到更大规模的结构、更逼真的模型和更高的维度。*(Iii)无序材料：我们在这里的目的是在基本水平上了解典型的非晶半导体a-Si的结构，特别是缺陷的性质及其对局域有序和弛豫的影响。我们预计，答案将来自a-Si和a-Ge的结构数据的详细比较，这些数据是最近从高能X射线衍射实验中获得的。这样的比较提供了数据中的“对比”，并将使人们对秩序的各种贡献有一些了解。我们将首先开发/优化a-Ge的模型，并将其与我们的a-Si模型进行比较。使用这两个模型，我们将详细说明一种方法，以更接近地模拟原始样品和退火样品之间的差异，这是一个尚未得到适当解决的问题。**

This research program aims at understanding how the atomic-scale structure of materials affects their properties on the larger scale, focusing on nanoscale systems, viz. nanoparticles, nanojunctions, etc. For this purpose, we resort to a variety of computational schemes that provide the relevant physics of the systems, in particular molecular dynamics simulations, and at the same time allow us to examine the effect of different parameters individually. Our research program will focus on three different themes: ***(i) Laser-matter interactions, in particular ablation of solid materials by ultrashort (several fs to hundreds of ps), intense laser pulses, a problem to which we have largely contributed in the past. We will in particular examine the problem of ablation and the subsequent production of nanoparticles in targets immersed in a liquid (typically water), a method routinely used for the production of nanoparticles, which are themselves used e.g. in medical applications. We will investigate the "manipulation" of the laser pulse so as to optimize the structure of the ejected matter and achieve tailor-made properties - a method referred to as pulse-shaping. Finally, we will examine the warm-dense matter and "plasma" regimes, where the physics is dominated by non-thermal processes, and in particular Coulomb effects (repulsion, explosion, charge effects). ***(ii) Nanothermics: We are interested in determining how heat is transported and dissipated in nanoscale materials, in particular semiconductors and carbon-based materials. For this purpose, we examine the phonon contributions to heat transport, i.e. how the various modes contribute individual heat currents and how they cross-correlate. We have found that these correlations act against heat current decay, which explains why one-dimensional materials are such good heat conductors. We will extend this work to larger scale structures, more realistic models, and higher dimensionalities. ***(iii) Disordered materials: Our aim here is to understand at the fundamental level the structure of the prototypical amorphous semiconductor a-Si, in particular the nature of defects and their influence on local order and relaxation. We expect that answers will come from a detailed comparison of structural data for a-Si with those for a-Ge which have recently become available from high energy x-ray diffraction experiments. Such a comparison provides "contrast" in the data and will give some understanding of the various contributions to order. We will first develop/optimize a model for a-Ge, which will be compared to our model for a-Si. Using these two models, we will elaborate a method for simulating more closely the difference between as-made and annealed samples, a problem that has not yet been properly addressed. **