Dispersion in water of two-dimensional (2D) nanosheets is conducive to their practical applications in fundamental science communities due to their abundance, low cost, and ecofriendliness. However, it is difficult to achieve stable aqueous 2D material suspensions because of the intrinsic hydrophobic properties of the layered materials. Here, we report an effective and economic way of producing various 2D nanosheets (h-BN, MoS, MoSe, WS, and graphene) as aqueous dispersions using carbon quantum dots (CQDs) as exfoliation agents and stabilizers. The dispersion was prepared through a liquid phase exfoliation. The as-synthesized stable 2D nanosheets based dispersions were characterized by UV-vis, HRTEM, AFM, Raman, XPS, and XRD. The solutions based on CQD decorated 2D nanosheets were utilized as aqueous lubricants, which realized a friction coefficient as low as 0.02 and even achieved a superlubricity under certain working conditions. The excellent lubricating properties were attributed to the synergetic effects of the 2D nanosheets and CQDs, such as good dispersion stability and easy-sliding interlayer structure. This work thus proposes a novel strategy for the design and preparation of high-performance water based green lubricants.

Sliding between two objects under very high load generally involves direct solid-solid contact at molecular/atomic level, the mechanism of which is far from clearly disclosed yet. Those microscopic solid-solid contacts could easily lead to local melting of rough surfaces. At extreme conditions, this local melting could propagate to the seizure and welding of the entire interface. Traditionally, the microscopic solid-solid contact is alleviated by various lubricants and additives based on their improved mechanical properties. In this work, we realized the state-of-the-art of extreme pressure lubrication by utilizing the high thermal diffusivity of liquid metal, 2 orders of magnitude higher than general organic lubricants. The extreme pressure lubrication property of gallium based liquid metal (GBLM) was compared with gear oil and poly-α-olefin in a four-ball test. The liquid metal lubricates very well at an extremely high load (10 kN, the maximum capability of a four-ball tester) at a rotation speed of 1800 rpm for a duration of several minutes, much better than traditional organic lubricants which typically break down within seconds at a load of a few kN. Our comparative experiments and analysis showed that this superextreme pressure lubrication capability of GBLM was attributed to the synergetic effect of the ultrafast heat dissipation of GBLM and the low friction coefficient of FeGa tribo-film. The present work demonstrated a novel way of improving lubrication capability by enhancing the lubricant thermal properties, which might lead to mechanical systems with much higher reliability.

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Given a large sample covariance matrix S N = 1 n Γ 1/2 N Z N Z * N Γ 1/2 N , where Z N is a N × n matrix with i.i.d. centered entries, and Γ N is a N × N deterministic Hermitian positive semidefinite matrix, we study the location and fluctuations of λmax(S N ), the largest eigenvalue of S N as N, n → ∞ and N n −1 → r ∈ (0, ∞) in the case where the empirical distribution µ Γ N of eigenvalues of Γ N is tight (in N ) and λmax(Γ N ) goes to +∞. These conditions are in particular met when µ Γ N weakly converges to a probability measure with unbounded support on R + .We prove that asymptotically λmax(S N ) ∼ λmax(Γ N ). Moreover when the Γ N 's are blockdiagonal, and the following spectral gap condition is assumed:where λ 2 (Γ N ) is the second largest eigenvalue of Γ N , we prove Gaussian fluctuations for λmax(S N )/λmax(Γ N ) at the scale √ n. In the particular case where Z N has i.i.d. Gaussian entries and Γ N is the N ×N autocovariance matrix of a long memory Gaussian stationary process (Xt) t∈Z , the columns of Γ