Second harmonic microscopy allows the detection of single atomic layers

2D crystals, a relatively new type of nanomaterial, have drawn considerable attention over the past 10 years.1 These subnanometer crystals are composed of single layers of atoms or molecules and include carbon (known as graphene), the transition metal dichalcogenides, boron nitride, and many others. They represent a new source of nanomaterials with exotic properties that can vary greatly from their 3D counterparts. Moreover, it is possible to use these 2D crystals as building blocks in the fabrication of new 3D crystals by stacking them in particular orders, and aligning them in certain relative orientations. This possibility opens up a new path toward designing and inventing functional materials for electronic, information, and energy technologies.2 Although laboratory research can rely on mechanical exfoliation to produce high quality 2D crystals from bulk (the so-called scotch-tapemethod), scalable techniques are necessary for future applications. These techniques include chemical vapor deposition, molecular beam epitaxy, and chemical exfoliation. To aid in their development, fast, noninvasive, and in situ diagnostic tools that can detect the quality of the 2D crystals are required. Optical microscopy can be used for this purpose, but its application is limited to identifying monolayer flakes. We have developed a nonlinear optical microscopy technique based on second harmonic generation (SHG). This technique can be used to identifymonolayers (single-layer samples) withmuch higher contrast ratios on most substrates. Because this process is extremely sensitive to crystal symmetry, it can distinguishmonolayers from bilayers (two-layer samples) with very high contrast. More importantly, the strong dependence of SHG on crystal orientation allows the crystal orientation of the sample to be Figure 1. Second harmonic power measured from molybdenum disulfide (MoS2) samples with different numbers of atomic layers. The contrast ratio between the substrate (0) and the monolayer (1) is at least 10,000, and the ratio between the monolayer and the bilayer (2) is about 1000.