Why $\rm Pt$-$\rm Ir$ Alloy or Tungsten is used for the tip in scanning tunneling microscopy? Just want to know the properties which qualify these materials to be used as the same.
 A: "Role of Tip Material in Scanning Tunneling Microscopy", C. Julian Chen, https://www.cambridge.org/core/journals/mrs-online-proceedings-library-archive/article/role-of-tip-material-in-scanning-tunneling-microscopy/B32182D2E29CDC6656348326D7AF243F :"In this paper, we show that atomic resolution in scanning tunneling microscopy (STM) originates from $p_z$ or $d_{z_2}$ states on the tip. Consequently, only a limited selection of tip materials can provide atomic resolution: $d$-band metals, for example, Pt, Ir, Pd, Rh, W, Mo; semiconductors that tend to form $p$-like dangling bonds, for example, Si."
See, in particular, Fig. 1 in this article.
EDIT (5/30/2019): Some additional relevant information from http://www2.cpfs.mpg.de/~ernst/ernst_da.pdf: "One commonly used tip material is platinum iridium (Pt-Ir). This alloy is particularly suitable for the use under ambient conditions because Pt is relatively inert to oxidation. A fraction of Ir is convenient to make the tip harder. Another advantage is the uncomplicated preparation. With some experience, cutting a piece of Pt-Ir wire with a punch or a pair of scissors (see ﬁgure 3.1) gives suﬃciently sharp tips to achieve atomic resolution. The success rate is rather low, though." Some weak points of the material are also discussed there.
A: For topography scanning purposes, especially for an undergrad experiment the pre-requisites are the robustness and stability in ambient conditions. Both PtIr and W make hard, and, with quite some practice, sharp tips which are ideal for STM.
Moreover, W tips, when chemically etched are the go-to in most STM labs [1].
[1] https://pubs.acs.org/doi/10.1021/nl010094q
