The difference of temperature in a material can convert heat energy into electrical energy. To harvest the heat energy, the material should have

This question is about thermoelectric materials (Seebeck effect), where a temperature difference is converted to electrical energy.

For efficient thermoelectric energy harvesting, the material should:

1. Have high electrical conductivity - to efficiently transport the generated electrical current with minimal resistive losses.

2. Have low thermal conductivity - to maintain the temperature difference across the material (if thermal conductivity is high, heat flows quickly and the temperature gradient disappears).

This is characterized by a high figure of merit $$ZT = \frac{S^2\sigma T}{\kappa}$$, where $$S$$ is the Seebeck coefficient, $$\sigma$$ is electrical conductivity, $$\kappa$$ is thermal conductivity, and $$T$$ is temperature.

A high ZT requires high electrical conductivity ($$\sigma$$) and low thermal conductivity ($$\kappa$$).

The correct answer is Option 4: low thermal conductivity and high electrical conductivity.