We use an ultrahigh-vacuum in-situ transmission electron microscope to study the behaviour of copper nanoparticles formed by inert-gas condensation onto unreactive substrates. We show that copper nanoparticles 'instantaneously' sinter upon contact in vacuo but that trace oxygen exposure slows this sintering and alters particle structures. These results support a picture where particles grow primarily by Brownian coagulation to produce a self-similar distribution. The model confirms the role of oxygen in inhibiting surface diffusion. The size distribution approaches the commonly reported log-normal distribution. Impurities such as oxygen may be desirable to limit agglomeration and to permit dense nanoparticle compact formation. [References: 37]
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