to fabricate the 
 vertical wse2/mos2 heterojunction 
 devices , wse2 was grown using a physical vapor deposition 
 process on a si / sio2 ( 300 nm sio2 ) substrate . a total of 0.2 g wse2 powder ( alfa aesar , 13084 ) was added 
 into an alumina boat as precursor . the blank si / sio2 substrates 
 ( 1 cm  5 cm ) were loaded into a home - built vapor deposition 
 system in a horizontal tube furnace ( lindberg / blue m ) with 1 in . the system was pumped down to a vacuum of 10 mtorr in 10 min , 
 and refilled with 150 sccm of ultrahigh purity argon gas ( airgas , 
  99.9999% ) then heated to desired growth temperature within 
 30 min . after that , the growth kept at the desired temperature for 
 30 min , and then terminated by shutting off the power of the furnace . the mos2 flakes were then exfoliated onto the wse2 flakes 
 through a micromechanical cleavage approach . the metal electrodes 
 ( for probe contact or wire bonding purposes ) were patterned on the 
 si / sio2 substrates by using electron - beam lithography and 
 electron - beam deposition of ti / au ( 50/50 nm ) thin film . ni / au ( 5/50 
 nm ) contact electrodes were then deposited to form the ohmic contact 
 to mos2 and au ( 50 nm ) was deposited to form the ohmic 
 contact to wse2 . the microstructures and morphologies of 
 the nanostructures are 
 characterized by a jeol 6700 scanning electron microscope ( sem ) . the 
 cross - sectional image of the heterostructure device is obtained by 
 an fei titan transmission electron microscope ( tem ) . the dc electrical 
 transport measurements were conducted with a lakeshore probe station 
 ( model ttp4 ) and a computer - controlled analogue - to - digital converter 
 ( national instruments model 6030e ) . the confocal micro - pl and raman 
 measurements were conducted on a horiba labhr confocal raman system 
 with 600 g / mm grating , 50 diffraction - limited objective ( n.a . = 0.75 ) , with an ar laser ( 514 nm ) or a he  ne laser ( 633 nm ) 
 excitation . the scanning photocurrent measurements were conducted 
 with the same horiba labhr confocal raman system combined with the 
 same electrical measurement system . the el measurements were performed 
 on a home - built confocal pl measurement system combining with the 
 same electrical measurement system with a temperature control in ar 
 environment . unless mentioned in the main text , all measurements were 
 conducted at room temperature . = 0.5 ) and captured by a liquid - nitrogen - cooled ccd 
 camera ( princeton instruments pylon 400f ) . the spectra were taken 
 by using an acton 2300i spectrometer with 150 g / mm grating and liquid - nitrogen - cooled 
 ccd .
the p 
 n diodes represent the most fundamental 
 device building blocks for diverse optoelectronic functions , but are 
 difficult to achieve in atomically thin transition metal dichalcogenides 
 ( tmds ) due to the challenges in selectively doping them into p- or n - type semiconductors . here , 
 we demonstrate 
 that an atomically thin and sharp heterojunction p 
 n diode can be created by vertically stacking p - type 
 monolayer tungsten diselenide ( wse2 ) and n - type few - layer molybdenum disulfide ( mos2 ) . 
 electrical 
 measurements of the vertically staked wse2/mos2 heterojunctions reveal excellent current rectification behavior 
 with an ideality factor of 1.2 . 
 photocurrent mapping shows rapid photoresponse 
 over the entire overlapping region with a highest external quantum 
 efficiency up to 12% . 
 electroluminescence studies show prominent band 
 edge excitonic emission and strikingly enhanced hot - electron luminescence . 
 a systematic investigation shows distinct layer - number dependent emission 
 characteristics and reveals important insight about the origin of 
 hot - electron luminescence and the nature of electron  orbital 
 interaction in tmds . 
 we believe that these atomically thin heterojunction p 
 n diodes represent an interesting system for probing 
 the fundamental electro - optical properties in tmds and can open up 
 a new pathway to novel optoelectronic devices such as atomically thin 
 photodetectors , photovoltaics , as well as spin- and valley - polarized 
 light emitting diodes , on - chip lasers .