Sorry to say but after doing B.Tech in Petroleum Engineering you are not eligible to do M.Tech in Nanotechnology because it requires B.Tech in relevant branch

But don’t worry you can do M.Tech in Petroleum Engineering which is also best course, and has great scope

M.Tech. Petroleum Engineering or Master of Technology in Petroleum Engineering is a postgraduate Petroleum Engineering course.

Eligibility for M.Tech Petroleum Engineering
Must have passed B.E. / B.Tech or equivalent degree in relevant branch with minimum 50% marks or equivalent CGPA

List of colleges offering M.Tech (Petroleum Engineering) in India
Aryabhatta Knowledge University - AKU
Chanakya National Law University Campus, Near Mithapur Bus Stand, Mithapur
Patna, Bihar
(0612) 2351919, 2362270
(0612) 2362270

Dibrugarh University
National Highway, Dibrugarh
Dibrugarh, Assam
(0373) 2370231, 2370336, 2370239, 2371040
9954480932, 9435331532, 8011050097
(0373) 2370768

Indian School of Mines University
Dhanbad, Jharkhand
(0326) 2235001, 2235004
(0326) 2296563

Pandit Deendayal Petroleum University - PDPU
Raisan Village
Gandhinagar, Gujarat
(079) 23275060
(079) 23275030

Rajiv Gandhi Institute of Petroleum Technology
Ratapur Chowk
Rae Bareli, Uttar Pradesh
(0535) 2217427, 2002593
(0535) 2211888, 2217456

Sri Sivani College of Engineering
Chilakapalem jn
Srikakulam, Andhra Pradesh
9441385616