LED characteristics to use in heating a material sample?
It really depends on what kind of sample you want to heat up, but the most common way would be to use IR LEDs, like these. I'd use a small lens to focus the beam on the sample (could be tuned precisely enough using visible light from red LED). Make sure you protect your eyes when you power up the IR LED though - its emission is invisible and doesn't make your iris contract or your eyes hurt, so it has potential to burn your retina when visible light would be mostly harmless.