  Perovskite SrVO$_3$ has recently been proposed as a novel electron emission
cathode material. Density functional theory (DFT) calculations suggest multiple
low work function surfaces and recent experimental efforts have consistently
demonstrated effective work functions of ~2.7 eV for polycrystalline samples,
both results suggesting, but not directly confirming, some fraction of even
lower work function surface is present. In this work, thermionic electron
emission microscopy (ThEEM) and high-field ultraviolet photoemission
spectroscopy are used to study the local work function distribution and measure
the work function of a partially-oriented-(110)-SrVO$_3$ perovskite oxide
cathode surface. Our results show direct evidence of low work function patches
of about 2.1 eV on the cathode surface, with corresponding onset of observable
thermionic emission at 750 $^o$C. We hypothesize that, in our ThEEM
experiments, the high applied electric field suppresses the patch field effect,
enabling the direct measurement of local work functions. This measured work
function of 2.1 eV is comparable to the previous DFT-calculated work function
value of the SrVO-terminated (110) SrVO$_3$ surface (2.3 eV) and SrO terminated
(100) surface (1.9 eV). The measured 2.1 eV value is also much lower than the
work function for the (001) LaB$_6$ single crystal cathode (~2.7 eV) and
comparable to the effective work function of B-type dispenser cathodes (~2.1
eV). If SrVO$_3$ thermionic emitters can be engineered to access domains of
this low 2.1 eV work function, they have potential to significantly improve
thermionic emitter-based technologies.
