Two-dimensional materials provide extraordinary opportunities for exploring phenomena arising in atomically thin crystals. Beginning with the first isolation of graphene, mechanical exfoliation has been a key to provide high-quality two-dimensional materials, but despite improvements it is still limited in yield, lateral size and contamination. Here we introduce a contamination-free, one-step and universal Au-assisted mechanical exfoliation method and demonstrate its effectiveness by isolating 40 types of single-crystalline monolayers, including elemental two-dimensional crystals, metal-dichalcogenides, magnets and superconductors. Most of them are of millimeter-size and high-quality, as shown by transfer-free measurements of electron microscopy, photo spectroscopies and electrical transport. Large suspended two-dimensional crystals and heterojunctions were also prepared with high-yield. Enhanced adhesion between the crystals and the substrates enables such efficient exfoliation, for which we identify a gold-assisted exfoliation method that underpins a universal route for producing large-area monolayers and thus supports studies of fundamental properties and potential application of two-dimensional materials.

The recent discovery of high-temperature superconductivity in iron-based compounds has attracted much attention. How to further increase the superconducting transition temperature (T_c) and how to understand the superconductivity mechanism are two prominent issues facing the current study of iron-based superconductors. The latest report of high-T_c superconductivity in a single-layer FeSe is therefore both surprising and significant. Here we present investigations of the electronic structure and superconducting gap of the single-layer FeSe superconductor. Its Fermi surface is distinct from other iron-based superconductors, consisting only of electron-like pockets near the zone corner without indication of any Fermi surface around the zone centre. Nearly isotropic superconducting gap is observed in this strictly two-dimensional system. The temperature dependence of the superconducting gap gives a transition temperature T_c~ 55 K. These results have established a clear case that such a simple electronic structure is compatible with high-T_c superconductivity in iron-based superconductors.

The latest discovery of high temperature superconductivity near 80 K in La₃Ni₂O₇under high pressure has attracted much attention. Many proposals are put forth to understand the origin of superconductivity. The determination of electronic structures is a prerequisite to establish theories to understand superconductivity in nickelates but is still lacking. Here we report our direct measurement of the electronic structures of La₃Ni₂O₇by high-resolution angle-resolved photoemission spectroscopy. The Fermi surface and band structures of La₃Ni₂O₇are observed and compared with the band structure calculations. Strong electron correlations are revealed which are orbital- and momentum-dependent. A flat band is formed from the Ni-3d $${}_{{z}^{2}}$$z2 orbitals around the zone corner which is ~ 50 meV below the Fermi level and exhibits the strongest electron correlation. In many theoretical proposals, this band is expected to play the dominant role in generating superconductivity in La₃Ni₂O₇. Our observations provide key experimental information to understand the electronic structure and origin of high temperature superconductivity in La₃Ni₂O₇.

The Kagome superconductors AV₃Sb₅ (A = K, Rb, Cs) have received enormous attention due to their nontrivial topological electronic structure, anomalous physical properties and superconductivity. Unconventional charge density wave (CDW) has been detected in AV₃Sb₅. High-precision electronic structure determination is essential to understand its origin. Here we unveil electronic nature of the CDW phase in our high-resolution angle-resolved photoemission measurements on KV₃Sb₅. We have observed CDW-induced Fermi surface reconstruction and the associated band folding. The CDW-induced band splitting and the associated gap opening have been revealed at the boundary of the pristine and reconstructed Brillouin zones. The Fermi surface- and momentum-dependent CDW gap is measured and the strongly anisotropic CDW gap is observed for all the V-derived Fermi surface. In particular, we have observed signatures of the electron-phonon coupling in KV₃Sb₅. These results provide key insights in understanding the nature of the CDW state and its interplay with superconductivity in AV₃Sb₅ superconductors.

The topological materials have attracted much attention for their unique electronic structure and peculiar physical properties. ZrTe₅ has host a long-standing puzzle on its anomalous transport properties manifested by its unusual resistivity peak and the reversal of the charge carrier type. It is also predicted that single-layer ZrTe₅ is a two-dimensional topological insulator and there is possibly a topological phase transition in bulk ZrTe₅. Here we report high-resolution laser-based angle-resolved photoemission measurements on the electronic structure and its detailed temperature evolution of ZrTe₅. Our results provide direct electronic evidence on the temperature-induced Lifshitz transition, which gives a natural understanding on underlying origin of the resistivity anomaly in ZrTe₅. In addition, we observe one-dimensional-like electronic features from the edges of the cracked ZrTe₅ samples. Our observations indicate that ZrTe₅ is a weak topological insulator and it exhibits a tendency to become a strong topological insulator when the layer distance is reduced.

The design and performance of the first vacuum ultraviolet (VUV) laser-based angle-resolved photoemission (ARPES) system are described. The VUV laser with a photon energy of $6.994\mathrm{eV}$ and bandwidth of $0.26\mathrm{meV}$ is achieved from the second harmonic generation using a novel nonlinear optical crystal $\mathrm{K}{\mathrm{Be}}_2\mathrm{B}{\mathrm{O}}_3{\mathrm{F}}_2$ . The new VUV laser-based ARPES system exhibits superior performance, including superhigh energy resolution better than $1\mathrm{meV}$ , high momentum resolution, superhigh photon flux, and much enhanced bulk sensitivity, which are demonstrated from measurements on a typical ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_8$ high temperature superconductor. Issues and further development related to the VUV laser-based photoemission technique are discussed.