Air Gap-Based Cavities Dramatically Enhance the True Intrinsic Spectral Signals of Suspended and Pristine Two-Dimensional Materials

The properties of two-dimensional (2D) materials are readily affected by their surroundings. Therefore, the underlying substrates and surrounding materials always disturb the pristine properties of 2D materials. Herein, we describe how the pristine properties of suspended 2D materials can be precisely extracted from Raman and photoluminescence (PL) spectra with great signal enhancements by taking advantage of both air gap suspension and nanocavity enhancement effects. The modes of the Raman emission lines were enhanced to almost the same degree when the 2D materials were positioned over the nanocavity: the 2D/G peaks of suspended single-layer graphene (SLG) and the E-2g(1)/A(1g) peaks of MoS2 were significantly enhanced almost equally. Moreover, recording Raman and PL spectra at different positions of the suspended 2D materials was a very powerful tool for observing charge transfer between the pristine 2D materials and the surrounding materials. We also found that the residual holes of the suspended SLG could be neutralized by aluminum (Al) at certain positions. By employing the air cavity structure, we could readily locate the charge neutrality point of the suspended 2D materials. In addition, the PL intensity of MoS2 could be greatly enhanced when using the same nanocavity. The great enhancements in the PL signals from the suspended 2D materials allowed us to further investigate the spectral weights of both the A(0) exciton and A(-) trion peaks when MoS2 was suspended or supported upon various metal films. This approach may open up new doors for techniques allowing precise characterization of abundant information from pristine and suspended 2D materials.