Hardware-based cryptography that exploits physical unclonable functions is required for the secure identification and authentication of devices in the Internet of Things. However, physical unclonable functions are typically based on anticounterfeit identifiers created from randomized microscale patterns or non-predictable fluctuations of electrical response in semiconductor devices, and the validation of an encrypted signature relies on a single-purpose method such as microscopy or electrical measurement. Here we report nanoscale physical unclonable function labels that exploit non-deterministic molecular self-assembly. The labels are created from the multilayer superpositions of metallic nanopatterns replicated from self-assembled block co-polymer nanotemplates. Due to the nanoscale dimensions and diverse material options of the system, physical unclonable functions are intrinsically difficult to replicate, robust for authentication and resistant to external disturbance. Multiple, independently operating keys—which use electrical resistance, optical dichroism or Raman signals—can be generated from a single physical unclonable function, offering millisecond-level validation speeds. We also show that our physical unclonable function labels can be used on a range of different surfaces including dollar bills, human hair and microscopic bacteria.

Ingenious solutions to reduce energy consumption and improve power efficiency are the need of the hour to tackle climate change. Radiative cooling is being extensively studied as a potential route for passive cooling and energy harvesting. However, to achieve almost complete solar reflection necessary for daytime sub-ambient cooling, the visual appearance of radiative coolers proposed so far has primarily been limited to being either diffusive white or mirror-like, which may be undesirable in real applications in terms of aesthetics, safety, and daytime light pollution. In this work, we demonstrate that optimally designed wavelength conversion in the visible spectral region allows the realization of radiative coolers with any desired color in the entire absorptive color space, including black and other dark colors, while still achieving daytime sub-ambient cooling. To obtain these results, we propose a general spectral design method based on metamerism that can find the ideal wavelength conversion spectrum with minimal solar absorption under strict color-matching constraints. Through our work, we provide guidelines for designing wavelength conversion profiles of suitable photoluminescent colorants and set performance boundaries for radiative coolers based on them.

The fabrication and characterization of nanoscale hole arrays (NHA) have been extensively performed for a variety of unique characteristics including extraordinary optical transmission phenomenon observed for plasmonic NHAs. Although the size miniaturization and hole densification are strongly required for enhancement of high-frequency optical responses, from a practical point-of-view, it is still not straightforward to manufacture NHA using conventional lithography techniques. Herein, a facile, cost-effective, and transferrable fabrication route for high-resolution and high-density NHA with sub-50 nm periodicity is demonstrated. Solvent-assisted nanotransfer printing with ultrahigh-resolution combined with block copolymer self-assembly is used to fabricate well-defined Si nanomesh master template with 4-fold symmetry. An Au NHA film on quartz substrate is then obtained by thermal-evaporation on the Si master and subsequent transfer of the sample, resulting in NHA structure having a hole with a diameter of 18 nm and a density over 400 holes/μm. A resonance peak at the wavelength of 650 nm, which is not present in the transmittance spectrum of a flat Au film, is observed for the Au NHA film. Finite-difference time-domain (FDTD) simulation results propose that the unexpected peak appears because of plasmonic surface guiding mode. The position of the resonance peak shows the sensitivity toward the change of the refractive index of surrounding medium, suggesting it as a promising label-free sensor application. In addition, other types of Au nanostructure arrays such as geometry-controlled NHA and nanoparticle arrays (NPAs) shows the outstanding versatility of our approach.

Transparent conducting electrodes (TCEs) are essential components in various optoelectronic devices. Nanostructured metallic thin film is one of the promising candidates to complement current metal oxide films, such as ITO, where high cost rare earth elements have been a longstanding issue. Herein, we present that multiscale porous metal nanomesh thin films prepared by bimodal self-assembly of block copolymer (BCP)/homopolymer blends may offer a new opportunity for TCE. This hierarchical concurrent self-assembly consists of macrophase separation between BCP and homopolymer as well as microphase separation of BCP, and thus provides a straightforward spontaneous production of a highly porous multiscale pattern over an arbitrary large area. Employing a conventional pattern transfer process, we successfully demonstrated a multiscale highly porous metallic thin film with reasonable optical transparency, electro-conductance, and large-area uniformity, taking advantage of low loss light penetration through microscale pores and significant suppression of light reflection at the nanoporous structures. This well-defined controllable bimodal self-assembly can offer valuable opportunities for many different applications, including optoelectronics, energy harvesting, and membranes.

We investigate the fundamental limit of radiative cooling of objects on the Earth's surfaces under general conditions including nonradiative heat transfer. We deduce the lowest steady-state temperature attainable and highest net radiative cooling power density available as a function of temperature. We present the exact spectral emissivity that can reach such limiting values, and show that the previously used 8–13 μm atmospheric window is highly inappropriate in low-temperature cases. The critical need for materials with simultaneously optimized optical and thermal properties is also identified. These results provide a reference against which radiative coolers can be benchmarked.