Pyroelectricity, a property of certain crystalline materials, is the creation of a temporary surface charge upon temperature change, resulting in an external electric current. Thermal movement of the molecules alters their average positions leading to changes in the crystal dipole moments. The Greek philosopher Theophrastus discovered this property in the mineral tourmaline already in 314 BC. Since then, this phenomenon has led to a plethora of applications, such as night vision devices, burglar alarms, and portable high-voltage generators. [1] The ability of pyroelectric crystals to develop two oppositely charged faces proves that they have spontaneous polarization, and therefore only crystals belonging to the ten polar crystallographic classes out of the 32 were considered to be pyroelectric. However, this assumes bulk polarization only and ignores the role of surfaces. Although all crystals including the nonpolar ones are delineated by 2D polar surfaces, [2] no pyroelectric current in an external circuit was previously reported from such surfaces since the number of molecules residing at these sites is too small to create sufficiently thick polar layers, to allow the accumulation of neutralizing depolarization charge. Accordingly, the pyroelectric properties of crystals were only correlated with the polarity of their mathematical lattices.As part of our studies on the conversion of nonpolar host crystals by stereospecific doping with guest molecules into polar pyroelectric mixed crystals, we discovered, surprisingly, that even the pure centrosymmetric crystals of a-glycine exhibit surface pyroelectricity from their {010} faces.When centrosymmetric a-glycine crystals, space group P2 1/n , are grown in pure water in the presence of l-alanine (Figure 1 a), the molecules of the latter are occluded enantio-selectively ( % 1 %) through the ð0 1 0Þ face and induce {010}plate-like crystals (Figure 1 b). [3,4] Consequently, weak bulk pyroelectricity is anticipated from such crystals. Surprisingly, upon step-like heating with a modulated laser (see the Supporting Information), both {010} faces of these 0.8-2 mm thick crystals generate substantial negative pyroelectric current corresponding to a 4-12 10 À11 C cm À2 K À1 pyroelectric coefficient (Figure 1 c, Table 1). This pyroelectric coefficient is only ten times smaller than those measured for common polar pyroelectric materials, such as tourmaline or the well-studied lithium perchlorate trihydrate. [5] However, the direction of the pyroelectric current generated at the ð0 1 0Þ and (010) faces upon heating is the same. This outcome and the rapid decay of the pyroelectric current after switching the laser on, are definitely inconsistent with a bulk pyroelectric effect that might arise from the entire crystal. [6] Other explanations attributable to the photo-or to the thermoelectric (Seebeck) effect, trapped charges, secondary pyroelectricity or the flexoelectric effect are inconsistent with the formation of this current (see the Supporting Information for detailed analy...
