I doubt you find any optical method with a resolving power that small due to the wavelength of light. 

By EDX you would need to work on the Mo L lines at 2.29 keV so your incident voltage would have to be around 5kV in order to excite it. 7nm is quite thin. I have made a simulation of the spectrum (guessing that the substrate is glass) in the file below. So theoretically yes it is possible to detect the Mo.

Thanks Ian,

I have to detect 7 nm thickness MoO3 experimentally. Any other method to find thickness??

Yo might want to look at the profiler instrumentation from Bruker in the links.

Also Bruker's XRF thin film measurement techniques.

There are some EDX applications now available that might do this such as LayerProbe from Oxford Instruments in the AZtec suite.

You might also consider using Grazing incidence XRD

https://www.bruker.com/fileadmin/user_upload/8-PDF-Docs/SurfaceAnalysis/StylusProfilometry/Brochures/B518-RevA0-Dektak_XTL_Stylus_Profiler_System-Brochure.pdf

https://www.bruker.com/products/x-ray-diffraction-and-elemental-analysis/x-ray-fluorescence/xrf-solutions/overview/xrf-solutions/mlplus.html

http://www.oxford-instruments.com/products/microanalysis/solutions/layerprobe

Thanks Ian Sir, I will go through this.

With much curiosity, thickness can be find by Grazing incidence XRD??